CN111458793A - LNOI-based ridge-type optical waveguide end-face coupling structure and its application - Google Patents

Abstract

An LNOI-based ridge-type optical waveguide end-face coupling structure and application thereof. The LNOI-based ridge-type optical waveguide end-face coupling structure comprises a substrate; an insulating layer; a core layer ridge region, a second waveguide core layer slab region, and a second waveguide core layer ridge region; and a third waveguide core layer. In the structure of the present invention, the ridge region of the second waveguide core layer and the flat plate partition layer of the second waveguide core layer are asynchronously reduced into a wedge-shaped structure, so that the wedge-shaped tip of the ridge region of the second waveguide core layer does not need to be small in size. The optical field is converted to the flat plate area for continuous transmission, and then the wedge-shaped tip leaks in the flat plate area and extends to the third waveguide core layer for continuous transmission, which reduces the difficulty of the preparation process of the small-sized structure; the structure of the invention can realize the high efficiency of the LNOI chip and the optical fiber coupling, which is conducive to the realization of large-scale photonic integration.

Description

LNOI-based ridge-type optical waveguide end-face coupling structure and its application

technical field

The invention belongs to the field of integrated optics, and more particularly relates to an LNOI-based ridge-type optical waveguide end-face coupling structure and its application.

Background technique

Lithium niobate crystal is a ferroelectric at room temperature with excellent electro-optic effect, piezoelectric effect, pyroelectric effect, ferroelectric effect and nonlinear optical properties, while lithium niobate on insulator (Lithium niobate on insulator, LNOI ) has the excellent properties of lithium niobate crystal, high single crystallinity, large refractive index contrast difference, strong light limiting ability, and is widely used in the research of various optoelectronic devices in integrated optics, such as optical couplers, filters, frequency Passive and active devices such as converters, resonators, electro-optic modulators. The coupling between optical fibers and waveguide devices is an important research content in integrated optics. With the research and application of single crystal lithium niobate thin film devices, the efficient coupling between optical fibers and LNOI devices has become an important issue. Commonly used The coupling methods are divided into two categories: surface grating coupling and end-face coupling. The waveguide grating coupler utilizes the diffraction effect of the periodically etched grating on the waveguide to realize the phase modulation of the grating diffraction field, thereby realizing the coupling between the optical fiber and the waveguide surface, but the grating effect. The design of the coupler requires complex theoretical calculations, it is difficult to prepare on the film, and it cannot be further adjusted after fabrication, and it is difficult to realize the airtight packaging of the waveguide device with the optical path of vertical coupling; while the traditional butt coupling directly connects the optical fiber to the waveguide end face. Alignment realizes coupling, with small alignment tolerance and low coupling efficiency, it is difficult to efficiently couple light into thin-film devices with large scale differences, and the coupling difficulty for small-sized ridge-type optical waveguides is even more difficult.

SUMMARY OF THE INVENTION

In view of this, one of the main purposes of the present invention is to propose an LNOI-based ridge-type optical waveguide end-face coupling structure and its application, so as to at least partially solve at least one of the above technical problems.

In order to achieve the above purpose, as an aspect of the present invention, an LNOI-based ridge-type optical waveguide end-face coupling structure is provided, including:

substrate;

an insulating layer disposed on the substrate;

The ridge-type waveguide region includes: a first waveguide core-layer slab region, which is arranged on the insulating layer and is a fixed-width portion; a first waveguide core-layer ridge-type region, which is arranged on the first waveguide core layer slab region and is a fixed-width portion ; a second waveguide core plate area, disposed on the insulating layer, in contact with the first waveguide core plate area, and is a reverse wedge-shaped part; and a second waveguide core layer ridge area, disposed in the first waveguide core layer plate On the second waveguide core layer slab region and the second waveguide core layer plate region, in contact with the first waveguide core layer ridge region, it is a reverse wedge-shaped part; and

The third waveguide core layer, whose refractive index is smaller than that of the ridge waveguide region and greater than that of air, is a fixed width portion.

As another aspect of the present invention, an application of the above-mentioned LNOI-based ridge-type optical waveguide end-face coupling structure in the field of integrated optics is also provided.

Based on the above technical solutions, the LNOI-based ridge-type optical waveguide end-face coupling structure and its application of the present invention have at least one of the following advantages over the prior art:

1. The structure of the present invention is prepared with a reverse wedge-shaped waveguide core layer structure, which can be selected as a wedge shape in a pure horizontal direction, a wedge shape in a pure vertical direction, or a wedge shape in both the horizontal direction and the vertical direction. The expansion of the optical field in the horizontal and vertical directions, the sub-micron optical mode field in the LNOI chip (LN film thickness < 1 micron) is expanded to achieve efficient matching with the micron optical field in the fiber, improving the coupling efficiency;

2. In the structure of the present invention, the ridge-shaped region of the second waveguide core layer and the flat-plate partition layer of the second waveguide core layer are asynchronously reduced into a wedge-shaped structure, so that the wedge-shaped tip of the ridge-shaped region of the second waveguide core layer does not need to be small in size. The light field can be converted to the flat plate area for continuous transmission, and then the wedge-shaped tip leaks in the flat plate area and extends to the third waveguide core layer for continuous transmission, which reduces the difficulty of the fabrication process of small-sized structures (small ridges);

3. In the structure of the present invention, the end face of the third waveguide core layer coupled with the end face of the optical fiber is coated with an anti-reflection film, which reduces the reflection loss of the optical field and further improves the coupling efficiency;

4. The structure of the present invention can realize high-efficiency coupling between the LNOI chip and the optical fiber, which is beneficial to realize large-scale photonic integration.

Description of drawings

1 is a schematic structural diagram of an LNOI-based ridge-type optical waveguide end-face coupling structure according to an embodiment of the present invention;

FIG. 2 is a plan view of FIG. 1 .

In the above figure, the reference symbols have the following meanings:

1-substrate; 2-insulating layer; 3-first waveguide core layer slab area; 4-first waveguide core layer ridge-type region; 5-second waveguide core layer ridge-type region; 6-second waveguide core layer slab region; 7- the third waveguide core layer; 8- anti-reflection coating.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

The invention utilizes the layered changes in the width and height of the core layer of the ridge optical waveguide, and coats the anti-reflection film on the coupling end of the third waveguide, so as to realize the horizontal and vertical expansion of the optical field during the transmission from the ridge waveguide to the optical fiber. At the same time, the geometric size of the wedge-shaped tip of the second waveguide ridge region is increased, which reduces the difficulty of fabrication of small-sized structures, improves the mode field matching between the LNOI waveguide and the optical fiber, reduces the optical coupling reflection loss, and finally realizes the ridge. The cross-scale high-efficiency end-face coupling between the sub-micron optical mode field of the optical waveguide and the micron optical fiber mode field is conducive to the realization of large-scale integration of optical paths.

The invention discloses an LNOI-based ridge-type optical waveguide end-face coupling structure, comprising:

substrate;

an insulating layer disposed on the substrate;

The ridge-type waveguide region includes: a first waveguide core-layer slab region, which is arranged on the insulating layer and is a fixed-width portion; a first waveguide core-layer ridge-type region, which is arranged on the first waveguide core layer slab region and is a fixed-width portion ; a second waveguide core plate area, disposed on the insulating layer, in contact with the first waveguide core plate area, and is a reverse wedge-shaped part; and a second waveguide core layer ridge area, disposed in the first waveguide core layer plate On the second waveguide core layer slab region and the second waveguide core layer plate region, in contact with the first waveguide core layer ridge region, it is a reverse wedge-shaped part; and

The third waveguide core layer, whose refractive index is smaller than that of the ridge waveguide region and greater than that of air, is a fixed width portion.

In some embodiments of the present invention, the ridge region of the second waveguide core layer and the flat plate region of the second waveguide core layer are not synchronously reduced into a wedge-shaped structure.

In some embodiments of the present invention, both the tip directions of the flat plate region of the second waveguide core layer and the opposite wedge-shaped portion of the ridge region of the second waveguide core layer point toward the coupling end of the LNOI chip and the optical fiber.

In some embodiments of the present invention, the decreasing direction of the wedge-shaped portion of the opposite wedge-shaped region of the flat plate region of the second waveguide core layer and the ridge region of the second waveguide core layer includes any one of a width direction or a height direction or a combination of both.

In some embodiments of the present invention, the material used for the substrate includes any one of lithium niobate, quartz or silicon;

In some embodiments of the present invention, the material used for the insulating layer includes silicon dioxide, and the thickness is 1 to 5 microns.

In some embodiments of the present invention, the material used in the ridge waveguide region includes lithium niobate;

In some embodiments of the present invention, the thickness of the ridge waveguide region is less than 1 micron.

In some embodiments of the present invention, the material used for the third waveguide core layer includes silicon dioxide.

In some embodiments of the present invention, the optical mode field areas of the first waveguide core ridge region and the first waveguide core flat region are both less than 1 square micrometer;

In some embodiments of the present invention, the diameter of the optical mode field at the end face of the third waveguide core layer is between 2.5 μm and 6 μm.

In some embodiments of the present invention, the end-face coupling structure further includes an anti-reflection film for reducing light field reflection, the anti-reflection film is disposed on the coupling end face of the fixed width region of the third waveguide core layer.

The invention also discloses the application of the above-mentioned LNOI-based ridge-type optical waveguide end-face coupling structure in the field of integrated optics.

In an exemplary embodiment, the present invention discloses an LNOI-based optical waveguide end-face coupling structure, comprising:

The flat plate region of the first waveguide core layer, whose material property is LN (lithium niobate) in the uppermost layer of the LNOI system, is a fixed width part;

The ridge-type region of the first waveguide core layer, whose material property is the LN of the uppermost layer of the LNOI system, is a fixed width part;

The second waveguide core plate area, whose material properties are the LN of the uppermost layer of the LNOI system, is an inverse wedge-shaped part;

The ridge region of the second waveguide core layer has the material property of the uppermost LN of the LNOI system, which is an inverse wedge-shaped part;

The third waveguide core layer, whose material property is that the refractive index is smaller than LN but larger than air, is a fixed width part;

Anti-reflection coating to reduce reflection of end-coupled light fields;

a substrate for supporting the overall thin film structure;

The insulating layer, the part between the substrate and the uppermost LN film.

Wherein, the insulating layer is adhered to the substrate, the LN film layer is adhered to the insulating layer, and the anti-reflection film is coated on the coupling end face of the third waveguide core layer;

Wherein, the substrate is usually lithium niobate, quartz or silicon material;

Among them, the insulating layer is usually made of silicon dioxide material, and the thickness is usually between 1-5 microns;

Among them, the LN layer thickness of the ultra-thin thin film device prepared by the uppermost layer of LNOI is less than 1 micron;

Wherein, the third waveguide core layer is a material whose refractive index is as close to air as possible to reduce reflection, and may be a SiO ₂ material;

Wherein, the tip direction of the reverse wedge-shaped portion points to the coupling end of the LNOI chip and the optical fiber;

Wherein, the initial ridge waveguide optical mode field area of the ridge region of the first waveguide core layer is less than 1 square micrometer;

Wherein, the mode field diameter (Mode Field Diameter, MFD) of the third waveguide end face is between 2.5-6 microns;

Among them, the single-mode fiber can be selected as a lens/tapered fiber, and the optical mode field diameter is between 2.5-6 microns.

Wherein, the flat plate region and the ridge region of the first waveguide core layer are the normal optical transmission waveguide part in the passive or active device on the LNOI chip;

The flat plate area of the second waveguide core layer can be reduced in both horizontal and vertical directions to form a wedge shape, so as to realize the expansion of the light field of the wedge-shaped tip in both directions;

The ridge region of the second waveguide core layer can be reduced in both the horizontal and vertical directions to form a wedge shape, so as to realize the expansion of the light field of the wedge-shaped tip in both directions;

The second waveguide core plate area and the second waveguide core ridge area are not synchronously reduced into a wedge shape, and the second waveguide core ridge area is reduced to a certain size so that the ridge part of the light field is converted to the second waveguide core layer plate You can continue to transmit in the area;

The optical mode field in the third waveguide core layer and the optical mode field of the single-mode fiber realize matching coupling.

The ridge region of the second waveguide core layer and the flat plate region of the second waveguide core layer are asynchronously reduced into a wedge-shaped structure, so that the wedge-shaped tip of the ridge region of the second waveguide core layer does not need to be small in size to convert the optical field to The flat plate area continues to transmit, and then the wedge-shaped tip leaks in the flat area and extends to the third waveguide core layer and continues to transmit, which reduces the difficulty of the fabrication process of small-sized structures (small ridges);

The matching degree of the optical mode field in the third waveguide core layer and the optical mode field of the single-mode fiber is increased, and the coupling efficiency is increased, and the coupling end face of the third waveguide core layer is coated with an anti-reflection coating, which reduces the reflection of the optical field, further The coupling efficiency is improved.

If the slab region and ridge region of the second waveguide core start to shrink into a wedge and end synchronously, the size of the wedge tip needs to be very small to simultaneously expand the light field leakage into the third waveguide core and continue to transmit, forming and Fiber-matched light field;

The flat area and the ridge area of the wedge-shaped structure of the ridge waveguide are completed in steps in the process, and the size of the tip is limited by the precision of pattern preparation. The smaller the size, the more difficult the preparation process and the lower the yield;

The ridge region of the second waveguide core layer is first reduced to a wedge shape, and then the flat plate region of the second waveguide core layer is also reduced to a wedge shape, and the ridge region can convert the light field to the flat plate region even when the size is not so small. Then the wedge-shaped tip leaks at the flat plate area and extends to the third waveguide core layer for further transmission, and finally achieves mode field matching with the optical fiber.

The above-mentioned LNOI is prepared with a reverse wedge-shaped waveguide core layer structure, which can be selected as a wedge shape in a pure horizontal direction, a wedge shape in a pure vertical direction, or a wedge shape in both the horizontal and vertical directions, so as to realize the optical field in the waveguide in the horizontal direction. and vertical expansion, the sub-micron optical mode field in the LNOI chip (LN film thickness < 1 micron) can be extended to achieve efficient matching with the micron optical field in the fiber and improve the coupling efficiency;

The ridge region of the second waveguide core layer and the flat plate region of the second waveguide core layer are asynchronously reduced into a wedge-shaped structure, so that the wedge-shaped tip of the ridge region of the second waveguide core layer does not need to be small in size to convert the light field to a flat plate Then the wedge-shaped tip leaks in the flat plate area and extends to the third waveguide core layer and continues to transmit, which reduces the difficulty of the fabrication process of small-sized structures (small ridges);

The end face of the third waveguide core layer coupled with the end face of the optical fiber is coated with an anti-reflection film, which reduces the reflection loss of the optical field, further improves the coupling efficiency, and is conducive to realizing large-scale photon integration.

The technical solutions of the present invention will be further described below through specific embodiments and accompanying drawings. It should be noted that the following specific embodiments are only for illustration, and the protection scope of the present invention is not limited thereto.

As shown in FIG. 1 and FIG. 2 , this embodiment provides an LNOI-based ridge-type optical waveguide end-face coupling structure, including a substrate 1, an insulating layer 2, a first waveguide core layer slab region 3, and a first waveguide core layer ridge Molded region 4 , ridged region 5 of the second waveguide core layer, slab region 6 of the second waveguide core layer, third waveguide core layer 7 , anti-reflection coating 8 .

The insulating layer 2 is on the substrate 1, and the LN film layer is on the insulating layer 2. Usually, the thickness of the ultra-thin film layer is less than 1 micron. A ridge-type optical waveguide is prepared on the uppermost LN film. The first waveguide core layer ridge-type region 4 Combined with the first waveguide core slab region 3, it shows the cross-sectional structure of normal optical field transmission. The second waveguide core ridge region 5 and the second waveguide core slab region 6 are layered asynchronously to form an inverse wedge-shaped structure. Towards the wedge-shaped part toward the coupling end of the LNOI chip and the single-mode fiber, the optical field is transmitted from the first waveguide core slab region 3 and the first waveguide core ridge region 4 to the second waveguide core ridge region 5 and the second waveguide core layer plate region 6, wherein the light field leaks into the second waveguide core layer plate region 6 at the tip of the second waveguide core layer ridge region 5 and continues to transmit, to the tip of the second waveguide core layer plate region 6 leakage diffuses to the second waveguide core layer plate region 6. The transmission continues in the three waveguide core layers 7, and finally the end face of the third waveguide core layer 7 is coupled with the optical fiber, and the optical field will follow the wedge shape (the above-mentioned simple horizontal wedge, simple vertical wedge and horizontal + vertical wedge) to carry out light in the corresponding direction The expansion of the mode field, so as to realize the conversion of the sub-micron optical mode field in the LNOI chip to the micron optical field in the fiber, and realize high-efficiency coupling; the flat area and the ridge area of the ridge waveguide wedge structure are step-by-step in process Completed, the size of the tip is limited by the precision of graphic preparation, the smaller the size, the more difficult the preparation process and the lower the yield;

The second waveguide core ridge region 5 and the second waveguide core slab region 6 are not synchronously reduced into a wedge shape in layers, so that the wedge-shaped tip of the second waveguide core ridge region 5 can be small in size. The optical field can be converted to the second waveguide core slab area 6 for continuous transmission, and then the wedge-shaped tip of the second waveguide core slab area 6 leaks and extends to the third waveguide core layer 7 for continued transmission, and finally achieves mode field matching with the optical fiber. , which reduces the difficulty of the preparation process of the small-scale structure (small ridge type) on the chip; in addition, in order to further improve the coupling efficiency between the chip and the fiber, the third waveguide core layer 7 and the fiber coupling end face are plated with antireflection The film 8 reduces the reflection loss of the optical mode field, which is beneficial to realize large-scale photonic integration.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above-mentioned specific embodiments are only specific embodiments of the present invention, and are not intended to limit the present invention. Within the spirit and principle of the present invention, any modifications, equivalent replacements, improvements, etc. made should be included within the protection scope of the present invention.

Claims (10)

1. An LNOI-based ridge-type optical waveguide end-face coupling structure, comprising: substrate; an insulating layer disposed on the substrate; The ridge-type waveguide region includes: a first waveguide core-layer slab region, which is arranged on the insulating layer and is a fixed-width portion; a first waveguide core-layer ridge-type region, which is arranged on the first waveguide core layer slab region and is a fixed-width portion ; a second waveguide core plate area, disposed on the insulating layer, in contact with the first waveguide core plate area, and is a reverse wedge-shaped part; and a second waveguide core layer ridge area, disposed in the first waveguide core layer plate On the second waveguide core layer slab region and the second waveguide core layer plate region, in contact with the first waveguide core layer ridge region, it is a reverse wedge-shaped part; and The third waveguide core layer, whose refractive index is smaller than that of the ridge waveguide region and greater than that of air, is a fixed width portion. 2 . The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1 , wherein the ridge-type region of the second waveguide core layer and the flat plate partition layer of the second waveguide core layer are asynchronously reduced into a wedge-shaped structure. 3 . 3 . The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1 , wherein the tip directions of the flat plate region of the second waveguide core layer and the reverse wedge-shaped portion of the ridge region of the second waveguide core layer are both oriented. 4 . Point to the coupling end of the LNOI chip and the fiber. 4 . The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1 , wherein the flat plate region of the second waveguide core layer and the wedge-shaped portion of the reverse wedge-shaped region of the ridge-type region of the second waveguide core layer The decreasing direction includes either the width direction or the height direction or a combination of both. 5. The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1, wherein the material used for the substrate comprises any one of lithium niobate, quartz or silicon; The insulating layer is made of silicon dioxide with a thickness of 1 to 5 microns. 6. The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1, wherein the material used in the ridge-type waveguide region comprises lithium niobate; The thickness of the ridge waveguide region is less than 1 micron. 7 . The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1 , wherein the material used for the third waveguide core layer comprises silicon dioxide. 8 . 8 . The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1 , wherein the optical mode field areas of the ridge-type region of the first waveguide core layer and the flat region of the first waveguide core layer are both less than 1 square. 9 . microns; The diameter of the optical mode field at the end face of the third waveguide core layer is between 2.5 microns and 6 microns. 9 . The LNOI-based ridge-type optical waveguide end-face coupling structure according to claim 1 , wherein the end-face coupling structure further comprises an anti-reflection film for reducing the reflection of the light field, and the anti-reflection film is arranged on the third ridge. 10 . on the coupling end face of the fixed width region of the waveguide core layer. 10. The application of the LNOI-based ridge-type optical waveguide end-face coupling structure according to any one of claims 1 to 9 in the field of integrated optics.

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