LNOI-based spot-size converter directly coupled with single-mode fiber

[0021] Next, one embodiment of the present invention will be further explained below, but the present invention is not limited to this embodiment, and any modification, equivalent replacement, improvement, etc. according to the spirit and principles of the present invention, should be included in this Within the protection range of the invention.

[0022] A LnOi-based model converter directly coupled to a single mode fiber in this embodiment is shown in FIG.

[0023] This embodiment is used in a single mode fiber and a LNOI chip coupled between the double-layer forward cone (I), the double-layer forward cone waveguide (3), the left end surface and the single mode optical fiber, reverse cone ( II) Output of lithium niobate waveguide (7) with LnOI chip waveguide, is an object to achieve a mold field transform between the large mode field of the single mode fiber and the small mold field of the LNOI chip. Single-mode fiber selection mode field diameter 10 μm standard single mode fiber, light into the double-mode fiber into the double-layer forward cone waveguide (3), the starting field distribution is in the double layer The upper conical waveguide region of the cone waveguide (3), from left to right at the same length, the upper tapered waveguide width is greater than the lower tapered waveguide until the upper tapered waveguide right end width is smaller than the lower tapered waveguide right end width In this process, the light field is gradually compressed to only the lower tapered waveguide (3) of the lower tapered waveguide (3) and the outer clasping waveguide of the reverse conical region (II) ( 4) Connect, and the design of the width remaining in the inverse tapered region (II) to maintain stable transmission in the structure, at which time the light field is distributed in the reverse conical region (II) The outer clamp waveguide (4), subsequently enters the lithium niobate waveguide (6), since the lithium niobate refractive index is higher than the external layer material refractive index, and thus as the lithium niobate waveguide (6) width gradually width The light field restriction is completed by the cladding to the core layer; the final light field no longer leaks among the reverse cone outer classes (4), the light field is completely limited to the lithium niobate waveguide (6) Among them, it is transmitted to the LnOi chip waveguide connected to the output banyan niobate waveguide (7), and the fiber optic to the LNOI chip is coupled.

[0024] The overall coupling loss is mainly composed of the following three aspects:

[0025] (1) Fetice reflection loss and fiber optic input mold columns of the end surface of the fiber end face and the mode of the optical fiber input module and the end surface of the coupler structure are adapted. The reason for the formation of Fresnel reflective loss is that the fiber end face and the end surface of the coupler have a refractive index difference, and a mixer structure end faces a certain refractive index of a refractive index or a matching liquid having a certain refractive index can effectively eliminate Fresnel. Reflection, a brief description as an example in TE mode, assuming that the optical fiber end refractive index is n 1 The end surface refractive index of the coupler structure is n 2 , Incident angle θ 1 , Refractive angle θ 2 , According to the Fresnel reflection law calculating the reflection coefficient According to the Snes Ear Formula and Energy Reflectance R = R 2 Get an energy reflectivity Considering the theoretical vertical incident, Assume that the refractive index of the enhancement film or matching liquid is n ar ,at this time It is not difficult to find At the time of reflection loss in Fresnel, theoretically completely eliminated. Mode mismatch loss cannot be completely eliminated, can only be reduced as possible, figure 2 (a), (b) reflect the two polarization modes of TM and TE, and the input fiber end face is distributed in the end surface of the optical fiber in the end face, and it is apparent that the higher the matching degree. The smaller the introduction of the loss, the match η can be formula Character, where E i Indicate fiber end face angle amplitude distribution, E j Indicates that the input area end face field amplitude distribution, calculates the matching mode in two polarization mode of TM and TE.

[0026] (2) Transmission loss, the transmission loss is the introduction of the mode conversion process, but is actually limited to the manufacturing process, the waveguide structure surface is generally not absolute smooth, so the waveguide side wall roughness during transmission Introducing scattering loss, scattering loss cannot be completely eliminated, and can only be reduced as much as possible by perfect etching process and by, for example, CMP polishing processes. image 3 The schematic diagram of mode conversion in both TM and TE two polarization mode is reflected, assuming that the beam propagates in the X direction, reflecting the section (XY) mode distribution of the structure at different positions, and can be very intuitive to observe the mold field by large to small. The conversion process.

[0027] (3) Alignment tolerance loss, mainly due to the misalignment between the end face of the fiber end surface and the end of the coupler, mainly from the three horizontal directions, and around X, winding Y, around Z three shafts. In six directions, this misalignment can be used to complete the nanoscale alignment between the optical fiber and the coupler structure, and the fiber positioning groove, spot welding curing, etc. can be used to assist optimization, basically elimination. Alignment tolerance loss.

[0028] The fabrication process of the present invention is relatively simple, fully feasible, primarily divided into a lithium niobate waveguide (5) in a reverse conical region (II), a lithium niobate waveguide (6), and outputs lithium niobate waveguide (7) The production and double-layer forward conical regions (I), and the inverse conical regions (II) are made of external cladding waveguide (4).

[0029] First introduce the input of lithium niobate waveguides (5) in the reverse conical region (II), a lithium niobate waveguide (6), and the production of lithium niobate waveguides (7), process flow charts are Figure 4 (a), in the LNOI wafer top, lithium niobate surface spin coating is negative electron beam exposure, via the lithium niobate waveguide (5), a lithium niobate waveguide (6), and output ruthenium Lithium lithium waveguide (7) pattern definition, the electron beam exposure, the sputtering of the mask is performed after development, then remove the superfine mask, to the production of the mask version, then use inductive coupling plasma (ICP) Etching technique is etched to etch lithium lithium niobate, etching to obtain a lithium niobate waveguide (5), a lithium niobate waveguide (6), and output a lithium niobate waveguide (7), and finally remove the mask.

[0030] Subsequent double-layer forward conical regions (I), the production, process flow charts of the inverse clamp waveguide (4) in the reverse cone region (II) Figure 4 (b), in the present embodiment, the material is selected from the inverse tapered tapered zone (I), and the material of the outer cladding waveguide (4) is selected from the inner cladding waveguide (II). The first layer SU-8 is directly copied on the wafer sheet in the previous step, and the lower tapered waveguide and reverse cone defined on the first layer SU-8 by a photolithography. The pattern (II) in the shape (II), the pattern is exposed by ultraviolet photolithography techniques, but does not develop, and then coat the first layer SU-8 on the first layer SU-8 coating, through the photolithography Define the upper tapered waveguide of the double-layer forward cone waveguide (3) on the second layer SU-8, exposure, developed and simultaneously obtains the upper and lower two-layer waveguide structure, and completes the double-layer forward cone. Zone (I), in the reverse cone (II).

[0031] It should be noted in the process that the inverse tapered zone (II) is used in the width and conical lithium niobate waveguide (6) tip width without more than 120 nm, and the double layer is tapered (I). The etch width of the upper tapered waveguide at the middle double-layer tapered waveguide (3) should not exceed 1 micron.

[0032] In summary, the invention proposed a direct coupled to a single mode fiber, correctly and feasible, and the CMOS process compatible with the current phase is mature, and has potential economic and application value. It is expected to be widely used in the integrated optical field.