A Lithium Niobate Spot Size Converter with Quartz Substrate

Abstract

The present invention discloses a quartz substrate lithium niobate mode spot converter, which relates to the fields of optical communication and microwave photonics. The present invention includes a quartz substrate layer, a silica cladding layer, a lithium niobate waveguide layer, an oxide interlayer and an oxide upper cladding layer. The lithium niobate waveguide layer is a three-layer stepped structure, and the cross-sections of the fiber coupling ends of the three-layer lithium niobate waveguide layers are all isosceles trapezoids. The cross-sections of the fiber coupling ends of the oxide interlayer and the oxide upper cladding layer are both provided with notches at the lower part and isosceles trapezoidal protrusions at the upper part, and the oblique side angles of the isosceles trapezoidal protrusions of the oxide interlayer and the oxide upper cladding layer and the isosceles trapezoidal cross-sections of the lithium niobate waveguide layer do not exceed 70°. The present invention can achieve high coupling and low transmission loss when optical signals are coupled between an integrated lithium niobate photonic chip and an optical fiber, and reduces the process processing difficulty, and has high practicability.

Description

Technical Field

[0001] The present invention relates to the fields of optical communication and microwave photonics, and in particular to a lithium niobate mode spot converter on a quartz substrate. Background Art

[0002] In recent years, the size of semiconductor microelectronic chips has approached the theoretical limit, and it has become increasingly difficult to break through the limitations of Moore's Law. Optoelectronic integrated chips, due to their huge transmission bandwidth and excellent low-power consumption characteristics, are considered revolutionary devices that can break through Moore's Law and will change the future chip pattern. Among many optical waveguide materials, lithium niobate (LiNbO3) crystals have received extensive attention due to their excellent electro-optic, acousto-optic, nonlinear characteristics, wide optical transparency window, and relatively stable physical and chemical properties. When transmitting optical signals, it is necessary to couple the optical signals from the external optical fiber into the optoelectronic chip, or couple the processed optical signals from the optoelectronic chip into the optical fiber. Currently, there are two common optical coupling methods: a mode spot converter based on edge coupling and a grating coupler based on vertical coupling. The method to reduce the end-face coupling loss is to use a mode spot converter based on edge coupling. The mode spot converter expands its mode field size by changing the waveguide size to match the mode field of the optical fiber. Compared with the grating coupler, the mode spot converter is in the same plane as the device, ensuring that the transmission directions of the optical fields of the optical fiber and the waveguide are the same, avoiding additional losses caused by different wave vector directions. Therefore, a higher coupling efficiency can be obtained, and the working bandwidth is relatively wide, the polarization-dependent loss is low, and subsequent chip packaging is also relatively convenient.

[0003] In the field of optoelectronic integrated chips, thanks to the development of microfabrication technology, it is becoming a reality to fabricate ultra-high-quality waveguides, microcavities, and nonlinear structures on the lithium niobate thin film platform. The integrated optoelectronic devices realized by using these structures have also achieved a substantial improvement in performance. The lithium niobate material has a fast electro-optic response ability, can realize fast phase modulation in the photonic chip, and has a low transmission loss in the communication band. Integrating the pump light source and the second harmonic generation device on the same platform can eliminate the input loss of the sum frequency process and show better performance.

[0004] The lithium niobate thin film waveguide has a small size, is easy to integrate, and has a large refractive index contrast. Therefore, it has a stronger optical confinement ability and lower loss. For the lithium niobate waveguide platform, a well-designed high-efficiency coupler can reduce the coupling loss. For coupling light into the chip, the inherent working mechanism of the grating coupler limits its working bandwidth, and its sensitivity to the polarization state and fiber alignment also limits its application. To overcome the limitations of the grating coupler, the mode spot converter has been developed in integrated lithium niobate photonics. The mode spot converter usually couples the light in the tapered optical fiber into the waveguide. The mode field diameter of the tapered optical fiber is about 2 μm. To match this mode field diameter, an inverted tapered waveguide is usually designed at the chip edge to minimize the back reflection.

[0005] Currently, there are relatively few reported spot-size converters based on lithium niobate. Among them, the research team led by Professor Xia Jinsong at Huazhong University of Science and Technology studied a double-layer spot-size converter based on lithium niobate, using the z-cut method. It uses an inverted conical spot-size converter for coupling between a laser and a single-mode waveguide of a lithium niobate thin film. However, the lithium niobate structure has high requirements for processing technology. The edge of the lithium niobate needs to be at 90° to the ground, which has become a processing difficulty. Summary of the Invention

[0006] In view of this, the present invention provides a lithium niobate spot-size converter on a quartz substrate to solve the problems of low coupling efficiency between an integrated lithium niobate photonic chip and an optical fiber and great process difficulty in the actual processing process when optical signals are coupled between them.

[0007] To solve the above problems, the technical solution of the present invention is as follows:

[0008] A lithium niobate spot-size converter on a quartz substrate includes a quartz substrate layer 1, a silica cladding layer 2, a lithium niobate waveguide layer 3, and an oxide upper cladding layer 5 arranged in sequence from bottom to top; the lithium niobate waveguide layer 3 is a three-layer stepped structure. The first-layer lithium niobate waveguide 301 of the lithium niobate waveguide layer 3 is closely attached to the silica cladding layer 2. The cross-section of the optical fiber coupling end of the first-layer lithium niobate waveguide 301 is an isosceles trapezoid, and the cross-section width remains the same in the direction from the optical fiber coupling end to the integrated chip coupling end; the second-layer lithium niobate waveguide 302 is divided into three parts, and the cross-sections of all three parts are isosceles trapezoids. Each part gradually becomes wider in the direction from the optical fiber coupling end to the integrated chip coupling end, and the hypotenuse angle of the isosceles trapezoid remains unchanged and is equal to the hypotenuse angle of the isosceles trapezoid of the cross-section of the first-layer lithium niobate waveguide 301. The first part 3021 of the second layer gradually becomes wider, and after the width is the same as that of the first-layer lithium niobate waveguide 301, it fuses and extends, and is attached to the silica cladding layer 2. The narrowest side of the second part 3022 of the second layer is the same as the widest side of the first part 3021 of the second layer, and the opening angle becomes larger. The narrowest side of the third part 3023 of the second layer is the same as the widest side of the second part 3022 of the second layer, and the opening angle further becomes larger; the third-layer lithium niobate waveguide 303 is divided into two parts, and the cross-sections of both parts are isosceles trapezoids. The hypotenuse angle of the isosceles trapezoid remains unchanged and is equal to the hypotenuse angle of the isosceles trapezoid of the cross-section of the first-layer lithium niobate waveguide 301. The first part 3031 of the third layer gradually becomes wider in the direction from the optical fiber coupling end to the integrated chip coupling end, and the length is the same as that of the second part 3022 of the second layer. The width of the second part 3032 of the third layer is the same as the widest side of the first part 3031 of the third layer, and the length is the same as that of the third part 3023 of the second layer.

[0009] Furthermore, an oxide interlayer 4 is also provided between the lithium niobate waveguide layer 3 and the oxide upper cladding layer 5.

[0010] Furthermore, the cross-sections of the fiber coupling ends of the oxide interlayer 4 and the oxide upper cladding 5 are both shaped with a notch at the lower part and an isosceles trapezoidal protrusion at the upper part; and the notch of the oxide upper cladding 5 corresponds to the isosceles trapezoidal protrusion of the oxide interlayer 4, and the notch of the oxide interlayer 4 corresponds to the cross-section of the lithium niobate waveguide layer 3.

[0011] Furthermore, the hypotenuse angles of the isosceles trapezoidal protrusions of the oxide interlayer 4 and the oxide upper cladding 5 and the isosceles trapezoid of the cross-section of the lithium niobate waveguide layer 3 do not exceed 70°, and at the same time, the narrowest width of the lithium niobate waveguides at both ends is not less than 500 nm.

[0012] Furthermore, the height of the isosceles trapezoidal protrusion of the oxide interlayer 4 does not exceed 700 nm, and the height of the isosceles trapezoidal protrusion of the oxide upper cladding 5 does not exceed 1.4 μm.

[0013] The present invention has the following beneficial effects compared with the background art:

[0014] 1. In the present invention, the lithium niobate waveguide layer adopts a three-layer stepped transmission model, which can reduce the transmission loss.

[0015] 2. In the present invention, the scheme of adding an oxide interlayer between the lithium niobate waveguide layer and the oxide upper cladding can significantly improve the coupling efficiency between the emitted light and the tapered fiber.

[0016] 3. In the present invention, the lithium niobate waveguide layer, the oxide interlayer and the oxide upper cladding all adopt hypotenuse etching with an angle not exceeding 70 degrees, which reduces the processing difficulty and has a higher tolerance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic cross-sectional structure diagram of the fiber coupling end of a lithium niobate mode spot converter with high coupling efficiency and low transmission loss in an embodiment of the present invention.

[0018] Figure 2 is a top view of the lithium niobate waveguide layer in an embodiment of the present invention.

[0019] Figure 3 is an axonometric view of the lithium niobate waveguide layer in an embodiment of the present invention.

[0020] Figure 4 is an axonometric view of the oxide interlayer in an embodiment of the present invention.

[0021] Figure 5 is an axonometric view of the oxide upper cladding in an embodiment of the present invention.

[0022] Figure 6 is a top view of the transmission mode field distribution of the embodiment of the present invention at 1550 nm.

[0023] Figure 7 It is a side view of the transmission mode field distribution of the embodiment of the present invention at 1550 nm.

[0024] Figure 8 It is a transmission mode field distribution diagram of the emitted light of the embodiment of the present invention at 1550 nm.

[0025] Figure 9 It is a transmission efficiency diagram of the embodiment of the present invention near the wavelength of 1550 nm. Detailed implementation manners

[0026] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation manners:

[0027] As Figures 1-3 shown, a quartz substrate lithium niobate mode spot converter includes a quartz substrate layer 1, a silica cladding layer 2, a lithium niobate waveguide layer 3, and a silicon oxynitride upper cladding layer 5 arranged in sequence from bottom to top; the lithium niobate waveguide layer 3 is a three-layer stepped structure, the first layer of lithium niobate waveguide 301 of the lithium niobate waveguide layer 3 is closely attached to the silica cladding layer 2, the cross-section of the fiber coupling end of the first layer of lithium niobate waveguide 301 is an isosceles trapezoid, and the cross-section width remains the same along the direction from the fiber coupling end to the integrated chip coupling end; the second layer of lithium niobate waveguide 302 is divided into three parts, the cross-sections of the three parts are all isosceles trapezoids, each part gradually becomes wider along the direction from the fiber coupling end to the integrated chip coupling end, the hypotenuse angle of the isosceles trapezoid remains unchanged, and is equal to the hypotenuse angle of the isosceles trapezoid of the cross-section of the first layer of lithium niobate waveguide 301. The first part 3021 of the second layer extends from 5 μm away from the fiber coupling end to the integrated chip coupling end, and the first part 3021 of the second layer gradually becomes wider. After the width is the same as that of the first layer of lithium niobate waveguide 301, it fuses and extends, and fits with the silica cladding layer 2. The second part 3022 of the second layer extends from 17 μm away from the fiber coupling end to the integrated chip coupling end, and the narrowest side of the second part 3022 of the second layer is the same as the widest side of the first part 3021 of the second layer, and the included angle becomes larger. The third part 3023 of the second layer extends from 27 μm away from the fiber coupling end to the integrated chip coupling end, with a total length of 18 μm, and the narrowest side of the third part 3023 of the second layer is the same as the widest side of the second part 3022, and the included angle further becomes larger; the third layer of lithium niobate waveguide 303 is divided into two parts, the cross-sections of the two parts are all isosceles trapezoids, the hypotenuse angle of the isosceles trapezoid remains unchanged, and is equal to the hypotenuse angle of the isosceles trapezoid of the cross-section of the first layer of lithium niobate waveguide 301. The first part 3031 of the third layer gradually becomes wider along the direction from the fiber coupling end to the integrated chip coupling end, and the length is the same as that of the second part 3022 of the second layer. The width of the second part 3032 of the third layer is the same as the widest side of the first part 3031 of the third layer, and the length is the same as that of the third part 3023 of the second layer.

[0028] Specifically, the height of the first layer lithium niobate waveguide 301 is 190 nm, and the short side length of the isosceles trapezoid cross-section is constant at 0.5 μm; the height of the second layer lithium niobate waveguide 302 increases to 0.3 μm. The short side length of the isosceles trapezoid cross-section of the first part 3021 of the second layer varies from 0.1 μm to 0.7 μm, the short side length of the isosceles trapezoid cross-section of the second part 3022 of the second layer increases from 0.7 μm to 3 μm, and the short side length of the isosceles trapezoid cross-section of the third part 3023 of the second layer increases from 3 μm to 8 μm; the height of the third layer lithium niobate waveguide 303 increases to 0.6 μm, and the short side length of the isosceles trapezoid cross-section of the first part 3031 of the third layer increases from 0.1 μm to 1.5 μm, and the short side length of the isosceles trapezoid cross-section of the second part 3032 of the third layer is constant at 1.5 μm.

[0029] Through the design of the three-layer stepped structure of the lithium niobate waveguide layer, the effect of low transmission loss is achieved.

[0030] Furthermore, as Figure 1 shown, a silica interlayer 4 is also provided between the lithium niobate waveguide layer 3 and the silicon oxynitride upper cladding 5.

[0031] Specifically, the quartz substrate layer 1 is used to support the entire chip. The thickness of the silica cladding 2 is 2 μm, and the refractive index is 1.47. The quartz substrate lithium niobate mode converter operates in the TE polarization mode; through the design of the silica interlayer 4, the coupling efficiency between the emitted light and the pulling optical fiber can be significantly improved.

[0032] Furthermore, as Figure 4 、 Figure 5 shown, the cross-sections of the fiber coupling ends of the silica interlayer 4 and the silicon oxynitride upper cladding 5 are both rectangular with a notch at the center of one side and an isosceles trapezoid protrusion at the center of the corresponding side; and the notch of the silicon oxynitride upper cladding 5 corresponds to the isosceles trapezoid protrusion of the silica interlayer 4, and the notch of the silica interlayer 4 corresponds to the cross-section of the lithium niobate waveguide layer 3.

[0033] Specifically, the heights of the rectangular part and the isosceles trapezoid part of the silica interlayer 4 are both 700 nm, the short side length of the isosceles trapezoid part is 1.4 μm, the heights of the rectangular part and the isosceles trapezoid part of the silicon oxynitride upper cladding 5 are both 1.4 μm, and the short side length of the isosceles trapezoid part is 6.5 μm.

[0034] Furthermore, the oblique angles of the isosceles trapezoid protrusions of the first oxide interlayer 4 and the second oxide upper cladding 5 and the hypotenuse angles of the isosceles trapezoid cross-section of the lithium niobate waveguide layer 3 are all 70°.

[0035] Through the design with an inclination angle not exceeding 70°, EBL is not required for processing, and photolithography can be used for photolithography, which can reduce the process difficulty and greatly save the process processing cost.

[0036] Specifically, as Figures 6-9 shown, the transmission mode field distribution diagram and transmission efficiency diagram at 1550 nm of this preferred embodiment are presented. It can be seen that most of the light propagates along the direction of the mode spot converter, and the light loss in other directions is less.

[0037] In summary, the present invention provides a lithium niobate mode spot converter on a quartz substrate, achieving high coupling and low loss when coupling optical signals between an integrated lithium niobate photonic chip and an optical fiber, and reducing the process processing difficulty.

[0038] Finally, it should be noted that the above are only the preferred embodiments of the present invention and do not limit the present invention. Any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and scope of the present invention defined by the appended claims shall be included within the protection scope of the present invention.

Claims

1. A quartz substrate lithium niobate mode spot converter, comprising a quartz substrate layer (1), a silica cladding layer (2), a lithium niobate waveguide layer (3) and an oxide upper cladding layer (5) which are sequentially arranged from bottom to top; characterized in that, The lithium niobate waveguide layer (3) has a three-layer stepped structure. The first layer of the lithium niobate waveguide (301) of the lithium niobate waveguide layer (3) is in close contact with the silica cladding (2). The cross-section of the fiber coupling end of the first layer of the lithium niobate waveguide (301) is an isosceles trapezoid, and the cross-section width remains the same in the direction from the fiber coupling end to the integrated chip coupling end. The second layer of the lithium niobate waveguide (302) is divided into three parts, and the cross-sections of the three parts are all isosceles trapezoids. Each part gradually widens in the direction from the fiber coupling end to the integrated chip coupling end, and the hypotenuse angle of the isosceles trapezoid remains unchanged and is equal to the hypotenuse angle of the isosceles trapezoid of the cross-section of the first layer of the lithium niobate waveguide (301). The first part (3021) of the second layer gradually widens, and after the width is the same as that of the first layer of the lithium niobate waveguide (301), it merges and extends and fits with the silica cladding (2). The narrowest side of the second part (3022) of the second layer is the same as the widest side of the first part (3021) of the second layer, and the opening angle becomes larger. The narrowest side of the third part (3023) of the second layer is the same as the widest side of the second part (3022) of the second layer, and the opening angle further becomes larger. The third layer of the lithium niobate waveguide (303) is divided into two parts, and the cross-sections of the two parts are all isosceles trapezoids. The hypotenuse angle of the isosceles trapezoid remains unchanged and is equal to the hypotenuse angle of the isosceles trapezoid of the cross-section of the first layer of the lithium niobate waveguide (301). The first part (3031) of the third layer gradually widens in the direction from the fiber coupling end to the integrated chip coupling end, and the length is the same as that of the second part (3022) of the second layer. The width of the second part (3032) of the third layer is the same as the widest side of the first part (3031) of the third layer, and the length is the same as that of the third part (3023) of the second layer.

2. A lithium niobate mode spot converter on a quartz substrate according to claim 1, wherein An oxide interlayer (4) is further provided between the lithium niobate waveguide layer (3) and the oxide upper cladding (5).

3. The spot-size converter of lithium niobate on quartz substrate according to claim 2, wherein The cross-sections of the fiber coupling ends of the oxide interlayer (4) and the oxide upper cladding (5) are both in the shape of having a notch at the lower part and an isosceles trapezoidal protrusion at the upper part; and the notch of the oxide upper cladding (5) corresponds to the isosceles trapezoidal protrusion of the oxide interlayer (4), and the notch of the oxide interlayer (4) corresponds to the cross-section of the lithium niobate waveguide layer (3).

4. The lithium niobate mode spot converter on a quartz substrate according to claim 3, wherein The hypotenuse angles of the isosceles trapezoidal protrusions of the oxide interlayer (4) and the oxide upper cladding (5) and the hypotenuse angle of the isosceles trapezoid of the cross-section of the lithium niobate waveguide layer (3) do not exceed 70°, and at the same time, the narrowest widths of the lithium niobate waveguides at both ends are not less than 500 nm.

5. A lithium niobate mode spot converter on a quartz substrate according to claim 3, characterized in that, The height of the isosceles trapezoidal protrusion of the oxide interlayer (4) does not exceed 700 nm, and the height of the isosceles trapezoidal protrusion of the oxide upper cladding (5) does not exceed 1.4 μm.

Images