CN113777711B - Large-mode-spot horizontal end face coupler based on lithium niobate film - Google Patents
Large-mode-spot horizontal end face coupler based on lithium niobate film Download PDFInfo
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- CN113777711B CN113777711B CN202110890825.5A CN202110890825A CN113777711B CN 113777711 B CN113777711 B CN 113777711B CN 202110890825 A CN202110890825 A CN 202110890825A CN 113777711 B CN113777711 B CN 113777711B
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/14—Mode converters
Abstract
The invention discloses a large-mode-spot horizontal end face coupler based on a lithium niobate thin film, and belongs to the technical field of integrated optical devices. The coupler comprises a double-layer inverted cone and a cladding waveguide; the double-layer inverted cone comprises an upper inverted cone and a lower inverted cone; the lower inverted cone material is silicon dioxide and lithium niobate, and the upper inverted cone material is lithium niobate; the cladding waveguide covers on the double-layer inverted cone, the cladding waveguide is made of silicon oxynitride, and the cross section of the cladding waveguide presents a dome convex shape. The lower inverted cone extends to the end face of the coupler chip; the upper inverted cone is cut off on the lithium niobate layer of the lower inverted cone. According to the large-spot horizontal end face coupler based on the lithium niobate thin film, the spot size of the coupler is increased and the coupling efficiency of the coupler is improved under the condition that an additional photoetching step is not added.
Description
Technical Field
The invention belongs to the technical field of integrated optical devices, and particularly relates to a large-spot horizontal end face coupler based on a lithium niobate thin film.
Background
The lithium niobate is insoluble in water, colorless and transparent, belongs to a trigonal system, is an ilmenite type (distorted perovskite type), and shows ferroelectricity, first-order and second-order electro-optic effects, piezoelectricity, photoelastic effect, photoinduced birefringence effect, photovoltaic effect (the forbidden band width is about 4eV) and the like. Lithium niobate crystals have a negative birefringence effect, n0 ═ 2.30 and ne ═ 2.21 (affected by chemical composition), and are transparent to electromagnetic waves having a wavelength from 350nm to 5200 nm. The lithium niobate crystal has a large linear electro-optic coefficient, and is suitable for preparing low-driving-voltage and high-speed electro-optic modulators and optical switches.
Lithium Niobate thin films on insulating Layers (LNOI) have attracted a great deal of attention in both academic and industrial circles. Due to its potential in ultra-high speed applications, this platform is considered one of the important candidates for the new generation of photonic integration platforms. Recently, a large number of LNOI-based devices have been reported and exhibit excellent performance. Such as low loss waveguides and high quality factor optical micro-ring resonators, tunable filters, high speed electro-optic modulators, optical frequency combs, second harmonic generation, wavelength converters, on-chip integrated spectrometers, and the like.
The mode size supported in an LNOI-based integrated waveguide is typically less than 1 μm, which is too far from standard-single-mode-fiber (SSMF), the mode spot size being approximately 10 μm, resulting in too large a direct fiber-to-integrated waveguide coupling scheme coupling loss. To achieve efficient fiber-to-chip optical coupling, an optical coupler is used. Solutions such as grating couplers (grating couplers), prism couplers (prism couplers) have been proposed, but grating couplers are limited by their low bandwidth and polarization dependence, and prism couplers require a stable mechanical pressure control structure to hold the prism in the correct position. Researchers have proposed a new type of optical coupler made of polymer using three-dimensional laser direct write (DLW) technology, which is based on three-dimensionally printed free-form surface microlenses connected to an on-chip waveguide by a Photonic Wire Bond (PWB). The mechanical and thermal stability of the suspended polymer structure is still poor.
The horizontal coupler has the advantages of high efficiency, broadband and irrelevant polarization, and has the characteristics of high stability and easy packaging, so that the application requirements of actual devices can be better met. In recent years, various LNOI-based horizontal end-face couplers have received attention. For example, a waveguide end facet covered with tantalum oxide is fabricated using Chemical Mechanical Polishing (CMP), but it is not compatible with a typical ridge waveguide. Research has proved that the ridge waveguide and the tapered lens fiber are matched through the mode of the single tapered ridge waveguide, but the coupling loss is still relatively high, and the double-layer tapered coupler can improve the performance, but the mode field distribution of the coupler controlled by the geometry of the tapered tip is different from that of the fiber. Therefore, in this structure, it is difficult to further improve the coupling efficiency. In 2020, y.pan et al reported a horizontal end-face coupler consisting of a double-layer tapered and clad waveguide (cldgg) consisting of a polymer with a coupling loss of 1.5dB per end-face. However, the presence of the polymer in the coupler structure brings mechanical instability and thermal instability, and limits the application scenarios. In addition, the distribution diameter of all mode fields of the edge coupler is less than 2.5 μm, and the size of the edge coupler is still far from that of a standard single-mode optical fiber.
Disclosure of Invention
In view of the above drawbacks or needs for improvement in the prior art, the present invention provides a large-spot horizontal end-face coupler based on a lithium niobate thin film, which aims to increase the size of the coupler spot and improve the coupling efficiency of the coupler without adding an additional photolithography step.
In order to achieve the purpose, the invention provides a large-mode-spot horizontal end face coupler based on a lithium niobate thin film, which comprises a double-layer inverted cone and a cladding waveguide;
the lower inverted cone material is silicon dioxide and lithium niobate, and the upper inverted cone material is lithium niobate;
the cladding waveguide cover in on the double-deck back taper, the awl point of double-deck back taper is located cladding waveguide center, the material of cladding waveguide is silicon oxynitride, and the cross-section of cladding waveguide presents the dome type of protruding font.
Further, the lower inverted cone extends to the end face of the coupler chip; the upper inverted cone is cut off on the lithium niobate layer of the lower inverted cone.
Further, the lower-layer inverted cone comprises a silicon dioxide oxygen-burying layer and a lithium niobate layer, wherein the inverted cone side wall of the silicon dioxide oxygen-burying layer is vertical and is consistent with the inverted cone shape of the lithium niobate layer.
Further, the coupler also includes a protective layer on the clad waveguide, the protective layer having a lower refractive index than the clad waveguide.
Further, the length-width ratio of the double-layer inverted cone is larger than or equal to 100, and the double-layer inverted cone has heat insulation properties.
Further, the cladding waveguide is grown by utilizing the conformal characteristic of chemical vapor deposition, and the number of extra photoetching processes is not increased.
Further, the section of the cladding waveguide is a dome ridge waveguide structure, and the diameter of the optical field mode spot is supported to be micron.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
(1) the double-layer inverted cone structure of the coupler is combined with the cladding waveguide, so that the on-chip efficient mode spot conversion is realized, the mode spot size can be effectively enlarged, the mode field distribution of the end face of the chip in the optical fiber is approximate, and the efficient optical coupling is realized;
(2) the double-layer inverted cone structure is prepared on the lithium niobate film and the oxygen buried layer, so that the problem of overhigh optical propagation loss of the single-layer tapered ridge waveguide can be effectively solved;
(3) the high-refractive-index lithium niobate structure in the coupler is positioned at the central position of the cladding waveguide, so that the coupling efficiency can be effectively improved, and the influence on the optical field distribution in the cladding waveguide is small due to the fact that the refractive index of the silicon dioxide buried oxide layer is close to that of silicon oxynitride;
(4) the invention utilizes the characteristic of conformal growth of Chemical Vapor Deposition (CVD), and directly forms the cladding waveguide without additional photoetching process step, thereby greatly reducing the process complexity, and the cladding waveguide has better mechanical and thermal stability.
Drawings
FIG. 1 is a top view of a structure according to an embodiment of the present invention;
FIG. 2 is a side view of a structure of an embodiment of the present invention;
FIG. 3 is a cross-sectional view of a structure of an embodiment of the present invention;
FIG. 4 is a cross-sectional view of a thin film lithium niobate provided by an embodiment of the present invention;
FIG. 5 is a schematic diagram of a process for fabricating a thin-film lithium niobate ridge waveguide and an upper-layer inverted cone according to an embodiment of the present invention;
fig. 6 is a schematic view of a process for manufacturing a lower-layer inverted cone of a thin-film lithium niobate ridge waveguide according to an embodiment of the present invention;
FIG. 7 is a schematic view of a process for fabricating an inverted cone of buried oxide silicon oxide according to an embodiment of the present invention;
FIG. 8 is a schematic diagram of a process for fabricating a clad waveguide according to an embodiment of the present invention;
FIG. 9 is a schematic diagram of a process for fabricating a silica cladding layer according to an embodiment of the present invention;
FIG. 10 is a schematic structural parameter diagram according to a first embodiment of the present invention;
FIG. 11 is a diagram of an electric field distribution according to a first embodiment of the present invention;
FIG. 12 is a schematic structural parameter diagram according to a second embodiment of the present invention;
fig. 13 is an electric field distribution diagram according to a second embodiment of the present invention.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
1-lithium niobate flat plate region, 2-silicon dioxide buried oxide layer, 3-upper lithium niobate waveguide and inverted cone, 4-lower lithium niobate inverted cone, 5-cladding waveguide, and 6-silicon dioxide cladding.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
First, terms related to the present invention are explained as follows:
PECVD: plasma Enhanced Chemical Vapor Deposition (PECVD).
ICP: inductively Coupled plasma etcher.
RIE: reactive Ion Etching, Reactive plasma etcher.
Burying an oxygen layer: a Buried Oxide Layer, an insulating Layer between two wafers.
Cladding waveguide: and (4) a clipping Waveguide structure with a larger characteristic mode area in the coupler.
As shown in fig. 1 and 2, a coupler according to an embodiment of the present invention includes:
upper lithium niobate waveguide and back taper 3
Lower lithium niobate inverted cone 4 positioned below the upper lithium niobate
And a silicon oxynitride clad waveguide 5 on the reverse taper
And a silica cladding 6 on the cladding waveguide
As shown in fig. 3, the silica buried oxide layer and the lower lithium niobate inverted cone are etched together to form an inverted cone structure with a large depth-to-width ratio, which can provide a basis for the conformal growth of the subsequent cladding waveguide, and meanwhile, the high-refractive-index lithium niobate structure is located at the center of the cladding waveguide, which can effectively improve the coupling efficiency, and because the silica refractive index of the buried oxide layer is close to the refractive index of silicon oxynitride, the influence on the optical field distribution in the cladding waveguide is small.
The working process of the horizontal end face coupler is as follows:
the signal light enters the end face of the coupler through the optical fiber to excite an intrinsic mode in a composite waveguide formed by the cladding waveguide, the inverted cone tip of the silica buried oxide layer and the inverted cone tip of the lithium niobate on the lower layer. And then the intrinsic mode is converted through a composite inverted cone structure formed by inverted cones of the silicon dioxide buried oxide layer and the lithium niobate at the lower layer, and the mode area is gradually reduced. And then, the mode area is further reduced through a composite inverted cone formed by the upper lithium niobate inverted cone and the lower lithium niobate inverted cone. And the final mode completely enters the lithium niobate ridge waveguide to complete the coupling process of light from the optical fiber to the lithium niobate ridge waveguide.
The length, the width and the thickness of each section of the upper-layer lithium niobate inverted cone, the lower-layer lithium niobate inverted cone and the silica buried oxide layer inverted cone are regulated and controlled simultaneously to ensure that the inverted cones have heat insulation property, so that efficient mode spot conversion is realized. The length of each segment of the inverted cone should be long enough (more than 100 micrometers) to maintain the gradual change and have the adiabatic property, and the embodiment of the present invention is not limited uniquely. The width of the tip of the double-layer inverted cone is the minimum value (about 200 nanometers) which can be reached by the current processing capacity, so that the light propagation loss between the inverted cone and the cladding waveguide is low enough, and the embodiment of the invention is not limited uniquely. The width of the cone bottom of the double-layer inverted cone is the minimum value capable of ensuring stable transmission of light in the inverted cone, and the width is adjusted according to the wavelength and the mode of light to be transmitted. The width and the etching depth of the lithium niobate waveguide on the upper layer of the coupler are determined by the number and the types of modes which are required to be supported by the on-chip optical waveguide, and the coupler can be designed according to actual requirements. The etching depth of the inverted cone of the silicon dioxide buried oxide layer is influenced by the total thickness of the buried oxide layer of the provided thin film lithium niobate substrate, the design is carried out according to actual conditions, and the embodiment of the invention is not limited uniquely. The shape and the size of the cladding waveguide are influenced by the conformal growth property and the growth time of the growth program of the lower lithium niobate inverted cone, the silica buried oxide layer inverted cone and the PECVD, the size of the cladding waveguide is close to the diameter of a fiber core of the optical fiber to be coupled, the cladding waveguide can be designed according to the type of the required coupling optical fiber, and the embodiment of the invention is not limited uniquely.
In the embodiment, the X-cut lithium niobate with the thickness of 500 nm is selected as the thin film lithium niobate, the total thickness of the buried oxide layer is 4.7 microns, and the substrate below the buried oxide layer is selected from silicon.
The embodiment comprises the following steps: the silicon dioxide back taper of the oxygen burying layer, the lithium niobate lower-layer back taper and the lithium niobate upper-layer back taper are formed, wherein the silicon dioxide back taper of the oxygen burying layer and the lithium niobate lower-layer back taper extend to the end face of the chip, and the lithium niobate upper-layer back taper is cut off on the lithium niobate lower-layer back taper.
In this embodiment, the side wall of the inverted cone of the silicon dioxide of the buried oxide layer is vertical and has the same shape as the inverted cone of the lower lithium niobate layer, and the embodiment of the present invention is not limited uniquely.
The double-layer inverted cone of the embodiment is sufficiently gradual and has heat insulation property, and the embodiment of the invention is not limited uniquely.
The cladding waveguide is grown by utilizing the conformal characteristic of the plasma enhanced chemical vapor deposition, and the number of extra photoetching processes is not increased. The refractive index of the cladding waveguide material is slightly higher than that of the silica of the buried oxide layer and that of the cladding silica, and the embodiment of the invention is not limited uniquely.
The cladding waveguide of the present embodiment has a morphology similar to a ridge waveguide, supporting a spot diameter of about 6.5 microns.
In this embodiment, the width of the double-layer inverted cone tip and the size of the cladding waveguide are simultaneously adjusted and controlled, and the size of the spot supported by the coupler is changed, so that the diameter of the spot is close to 6.5 micrometers.
In this embodiment, the protective layer has a lower refractive index than the cladding waveguide and is made of silica.
The invention also provides a preparation method of the coupler, which comprises the following steps:
s1, after cleaning a substrate, etching lithium niobate by ICP or RIE by using chromium as a hard mask to form an upper lithium niobate inverted cone and a waveguide;
s2, repeating the steps to form a lower-layer lithium niobate inverted cone;
s3, etching the silicon dioxide to form a silicon dioxide inverted cone of the buried oxide layer by continuously utilizing ICP or RIE;
and S4, growing a silicon oxynitride cladding waveguide and a silicon dioxide cladding by PECVD.
As shown in fig. 4, the lithium niobate slab region 1 may be etched by an etching technique such as ICP or RIE using chromium as a mask to form an upper lithium niobate inverted cone and a waveguide 3.
As shown in fig. 5, the lower lithium niobate inverted cone 4 may be formed by etching the lithium niobate flat plate region 1 by an etching technique such as ICP or RIE using chromium as a mask.
As shown in fig. 6, the silicon dioxide may be etched by an etching technique such as ICP or RIE using the chrome as a mask to form a buried oxide silicon dioxide back taper.
As shown in fig. 7, a silicon oxynitride clad waveguide can be grown conformally on the reverse taper by epitaxial techniques such as PECVD.
As shown in fig. 8, a silica cladding layer may be grown on the cladding waveguide by epitaxial techniques such as PECVD.
All the above patterning can be performed by EBL exposure.
The coupler for TE polarized light with 1550nm wavelength is taken as an example to show the coupling effect after the parameters are optimized. The cross-sectional view and key parameters of the coupler structure cladding waveguide are shown on the left side of fig. 10, and the top view and key parameters of the coupler back taper structure are shown on the right side of fig. 10. The total height was set to 6.5 μm, the ridge width was 6.5 μm, the taper height was 3.5 μm, the taper width one was 0.26 μm, the taper width two was 1.5 μm, the taper width three was 1.8 μm, the taper width four was 4 μm, the waveguide width was 0.9 μm, the taper length one was 100 μm, the taper length two was 50 μm, the taper length three was 50 μm, the lower layer reverse taper height was 0.24 μm, and the upper layer reverse taper height was 0.26 μm. FIG. 11 is a top view and a side view of the electric field distribution of the coupler. The total coupling loss when coupled with a polarization maintaining fiber having a spot diameter of 6.5 μm was 0.68dB, corresponding to a coupling efficiency of 86%.
The coupler for the TE polarized light with 1310nm wavelength is taken as an example below to show the coupling effect after the parameters are optimized. The cross-sectional view and the key parameters of the coupler structure cladding waveguide are shown on the left side of fig. 12, and the top view and the key parameters of the coupler inverted cone structure are shown on the right side of fig. 12. The total height was set to 6.5 μm, the ridge width was 6.5 μm, the taper height was 3.5 μm, the taper width one was 0.26 μm, the taper width two was 0.6 μm, the taper width three was 1.5 μm, the taper width four was 4 μm, the waveguide width was 0.9 μm, the taper length one was 400 μm, the taper length two was 50 μm, the taper length three was 50 μm, the lower layer reverse taper height was 0.24 μm, and the upper layer reverse taper height was 0.26 μm. FIG. 13 is a top view and a side view of the electric field distribution of the coupler. The total coupling loss when coupled to a polarization maintaining fiber having a 6.5 μm spot diameter was 0.49dB, corresponding to a coupling efficiency of 89%.
It will be appreciated by those skilled in the art that the foregoing is only a preferred embodiment of the invention, and is not intended to limit the invention, such that various modifications, equivalents and improvements may be made without departing from the spirit and scope of the invention.
Claims (6)
1. A large-mode-spot horizontal end face coupler based on a lithium niobate thin film is characterized in that the coupler comprises a double-layer inverted cone and a cladding waveguide;
the double-layer inverted cone comprises an upper inverted cone and a lower inverted cone; the lower inverted cone material is silicon dioxide and lithium niobate, and the upper inverted cone material is lithium niobate;
the cladding waveguide covers the double-layer inverted cone, the cone tip of the double-layer inverted cone is positioned in the center of the cladding waveguide, the cladding waveguide is made of silicon oxynitride, and the cross section of the cladding waveguide is in a dome convex shape;
the cladding waveguide is grown by utilizing the conformal characteristic of chemical vapor deposition, and the number of extra photoetching processes is not increased.
2. The lithium niobate thin film-based large-mode-spot horizontal end-face coupler of claim 1, wherein the lower inverted cone extends to the coupler chip end-face; the upper inverted cone is cut off on the lithium niobate layer of the lower inverted cone.
3. The lithium niobate thin film-based large-mode-spot horizontal end-face coupler of claim 2, wherein the lower inverted cone comprises a silica buried oxide layer and a lithium niobate layer, wherein an inverted cone sidewall of the silica buried oxide layer is vertical and is in accordance with an inverted cone shape of the lithium niobate layer.
4. The lithium niobate thin film-based large-mode-size horizontal end-face coupler of claim 1, further comprising a protective layer on the clad waveguide, the protective layer having a lower refractive index than the clad waveguide.
5. The lithium niobate thin film-based large-mode-spot horizontal end-face coupler of claim 1, wherein the aspect ratio of the double-layer inverted cone is equal to or greater than 100, and the double-layer inverted cone has thermal insulation properties.
6. The lithium niobate thin film-based large-mode-spot horizontal end-face coupler of claim 1, wherein the cladding waveguide cross-section is a dome ridge waveguide structure supporting optical field mode spot diameter up to micron level.
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| CN115808736A (en) * | 2021-09-15 | 2023-03-17 | 华为技术有限公司 | Optical coupler, optical chip and optical communication device |
| CN115144965A (en) * | 2022-07-29 | 2022-10-04 | 北京航空航天大学 | Lithium niobate thin film ridge waveguide end face coupler and preparation method thereof |
| CN116381852B (en) * | 2023-02-22 | 2023-10-17 | 武汉安湃光电有限公司 | Lithium niobate horizontal end face coupler capable of bearing high-power optical input |
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