Conical polarization beam splitter based on slit waveguide and preparation method thereof

Abstract

The present invention provides a tapered polarization beam splitter based on a slit waveguide and a preparation method thereof, wherein the beam splitter comprises a tapered coupler and a polarization filter; the tapered coupler comprises a strip silicon waveguide and a slit waveguide, and the strip silicon waveguide and the slit waveguide have opposite width variation trends in the coupling length direction; the polarization filter comprises a first polarization filter connected in series to the TE output end of the tapered coupler and a second polarization filter connected in series to the TM output end of the tapered coupler, the first polarization filter is used for conducting the TE polarization state and coupling the TM polarization state to the first slit waveguide; the second polarization filter is used for filtering the TE polarization state and converting the polarization mode of the TM polarization state from the second slit waveguide to the second strip silicon waveguide. The device of the present invention can be prepared by an integrated process, the process is simple and the tolerance is large, the size is small, a high extinction ratio can be achieved in a wide wavelength range, and integration with other devices is easy to achieve.

Description

Technical Field

[0001] The invention belongs to the field of optical devices and optical communications, and in particular relates to a tapered polarization beam splitter based on a slit waveguide and a preparation method thereof. Background Art

[0002] Polarization refers to the phenomenon that the vibration vector of a transverse wave (perpendicular to the propagation direction of the wave) is biased in certain directions. Polarization control plays a very critical role in many application fields, such as communications, biosensing, quantum optics, etc., and high-efficiency and small-size polarization control devices have very important application value in these fields.

[0003] In long-distance optical communications, optical fiber, as a transmission channel for optical signals, has poor polarization-maintaining capability, and the polarization state has great randomness. In silicon-based optoelectronics, since the propagation loss of the TE polarization state is low, devices are generally designed based on this polarization state. In addition, the effective refractive index of the TE and TM polarization modes of common silicon waveguides differs greatly, and no conversion between different polarization states occurs between the two, which has good polarization-maintaining capability. Therefore, when the optical signal enters the silicon photonic chip from the optical fiber, the polarization state of the input signal needs to be controlled.

[0004] The polarization beam splitter in optical communication is an integrated optoelectronic device used to separate the TE (transverse electric) mode and the TM (transverse magnetic) mode. Currently, polarization beam splitters based on couplers are widely used. However, most of the polarization beam splitters based on couplers have a small working bandwidth and are sensitive to the deviation of the size in actual processing, making them difficult to be used in practice. Summary of the invention

[0005] In view of the above-mentioned shortcomings of the prior art, the object of the present invention is to provide a conical polarization beam splitter based on a slit waveguide and a preparation method, so as to realize a polarization beam splitter with a larger tolerance, smaller size and higher extinction ratio in a wider wavelength range.

[0006] To achieve the above-mentioned purpose and other related purposes, the present invention provides a tapered polarization beam splitter based on a slit waveguide, the tapered polarization beam splitter comprising: a tapered coupler and a polarization filter connected in series; the tapered coupler comprises a strip silicon waveguide and a slit waveguide, the strip silicon waveguide and the slit waveguide have opposite width variation trends in the coupling length direction, the strip silicon waveguide has a width that gradually decreases as the coupling distance increases, and the slit waveguide has a width that gradually increases as the coupling distance increases; the polarization filter comprises a first polarization filter connected in series at a TE output end of the tapered coupler and a second polarization filter connected in series at a TM output end of the tapered coupler, the first polarization filter comprises a first strip silicon waveguide and a first slit waveguide, for conducting a TE polarization state, and coupling the TM polarization state into the first slit waveguide next to the first strip silicon waveguide to couple and filter out the TM polarization state; the second polarization filter comprises a second strip silicon waveguide and a second slit waveguide, for filtering the TE polarization state, and converting the polarization mode of the TM polarization state from the second slit waveguide to the second strip silicon waveguide.

[0007] Optionally, the slot waveguide includes a silicon lower layer, a silicon dioxide middle layer, and a silicon nitride upper layer stacked in sequence.

[0008] Furthermore, the width of the silicon nitride upper layer is greater than the width of the silicon lower layer.

[0009] Optionally, the strip silicon waveguide and the slot waveguide are also coated with a silicon dioxide upper cladding layer.

[0010] Optionally, a silicon dioxide filling layer is further filled between the strip silicon waveguide and the slot waveguide.

[0011] Optionally, the widths of the strip silicon waveguide and the slot waveguide of the tapered coupler change at the same rate, and the spacing between the strip silicon waveguide and the slot waveguide remains unchanged.

[0012] Optionally, the first strip silicon waveguide and the second strip silicon waveguide in the decoupling region of the polarization filter are bent in an S-shape to achieve a gradually increasing distance between the waveguides.

[0013] Optionally, the tapered coupler and the strip silicon waveguide and the slit waveguide of the polarization filter meet the phase matching condition of the TM polarization state.

[0014] The present invention also provides a method for preparing a tapered polarization beam splitter based on a slit waveguide, the method comprising the steps of: 1) providing an SOI substrate, and forming the strip silicon waveguide and the silicon lower layer of the slit waveguide in the SOI substrate by photolithography and etching; 2) forming a silicon dioxide middle layer of the slit waveguide on the silicon lower layer; 3) forming a silicon nitride upper layer of the slit waveguide on the silicon dioxide middle layer; 4) forming a silicon dioxide upper cladding on the strip silicon waveguide and the slit waveguide.

[0015] Optionally, step 2) uses a PECVD method to form a silicon dioxide middle layer of the slot waveguide on the silicon lower layer; step 3) uses an LPCVD method to deposit a silicon nitride layer, and forms a silicon nitride upper layer of the slot waveguide on the silicon dioxide middle layer through a photolithography and etching process, and the width of the silicon nitride upper layer is greater than the width of the silicon lower layer.

[0016] As described above, the tapered polarization beam splitter based on the slit waveguide and the preparation method of the present invention have the following beneficial effects:

[0017] In the present invention, by introducing a tapered structure into the coupler, the coupling of the TM polarization state is no longer very sensitive to the wavelength and the size deviation of the device, thereby improving the actual working bandwidth of the device and the ability to tolerate process errors in the actual manufacturing process.

[0018] In order to further improve the extinction ratio, the present invention uses two polarization filters connected in series at the output port of the tapered coupler, and both of them meet the phase matching conditions for the TM polarization state. The polarization filter at the TE output port of the tapered coupler can filter out the TM polarization state that is not fully coupled to the slit waveguide, and the filter at the TM output port can couple the TM in the original slit waveguide to the strip silicon waveguide. The filter not only realizes the function of filtering out the unnecessary TE polarization state, but also realizes the conversion of the TM polarization state from the slit waveguide to the conventional strip silicon waveguide, thereby avoiding the introduction of additional mode converters. The device of the present invention can realize the on-chip polarization splitting function, and can separate and obtain a relatively pure polarization state, and has many potential applications in future polarization multiplexing and sensing.

[0019] The device of the present invention can be prepared by using an integrated process, the process is simple and has a large tolerance, the size is small, a high extinction ratio can be achieved in a wide wavelength range, and integration with other devices is easy to achieve. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 to Figure 4 The structure diagram of the tapered polarization beam splitter based on the slit waveguide of the present invention is shown, wherein: Figure 2 Display as Figure 1 Schematic diagram of the cross-sectional structure at A-A', Figure 3Display as Figure 1 Schematic diagram of the cross-sectional structure at B-B'. Figure 4 Display as Figure 1 Schematic diagram of the cross-sectional structure at C-C'.

[0021] Figure 5 It is a schematic flow chart showing the steps of the method for preparing a tapered polarization beam splitter based on a slit waveguide according to the present invention.

[0022] Component number description

[0023] 1 Conical coupler

[0024] 11 Strip Silicon Waveguide

[0025] 12 Slot waveguide

[0026] 2 Polarization filter

[0027] 21 The first silicon waveguide

[0028] 22 First Slit Waveguide

[0029] 23 Second Strip Silicon Waveguide

[0030] 24 Second Slit Waveguide

[0031] 301 bottom silicon

[0032] 302 Insulation layer

[0033] 401 Silicon Sublayer

[0034] 402 Silicon dioxide intermediate layer

[0035] 403 Silicon Nitride Upper Layer

[0036] S11~S14 Step 1)~Step 4) DETAILED DESCRIPTION

[0037] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0038] For example, when describing the embodiments of the present invention in detail, for the sake of convenience, the cross-sectional view showing the device structure will not be partially enlarged according to the general scale, and the schematic view is only an example, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional space dimensions of length, width and depth should be included.

[0039] For ease of description, spatially relative terms such as "under", "below", "below", "below", "above", "on", etc. may be used herein to describe the relationship of one element or feature shown in the drawings to other elements or features. It will be understood that these spatially relative terms are intended to encompass other orientations of the device in use or operation in addition to the orientation depicted in the drawings. In addition, when a layer is referred to as being "between" two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present.

[0040] In the context of the present application, a structure in which a first feature is described as being "above" a second feature may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features are formed between the first and second features, such that the first and second features may not be in direct contact.

[0041] It should be noted that the illustrations provided in this embodiment are only used to illustrate the basic concept of the present invention in a schematic manner, and therefore the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0042] like Figure 1 to Figure 4 As shown, Figure 2 Display as Figure 1 Schematic diagram of the cross-sectional structure at A-A', Figure 3 Display as Figure 1 Schematic diagram of the cross-sectional structure at B-B'. Figure 4 Display as Figure 1 Schematic diagram of the cross-sectional structure at C-C' in the figure, this embodiment provides a tapered polarization beam splitter based on a slit waveguide, in particular, a tapered polarization beam splitter based on a silicon nitride / silicon slit waveguide, the tapered polarization beam splitter includes a tapered coupler 1 and a polarization filter 2 connected in series. In this embodiment, the tapered coupler 1 and the polarization filter 2 of the tapered polarization beam splitter are prepared based on an SOI substrate, the SOI substrate includes a bottom silicon 301, an insulating layer 302 and a top silicon layer, and the strip silicon waveguide and the lower silicon layer in the slit waveguide in this embodiment are both prepared based on the top silicon layer.

[0043] like Figure 1 , Figure 2 and Figure 3 As shown, the tapered coupler 1 includes a strip silicon waveguide 11 and a slot waveguide 12. The strip silicon waveguide 11 and the slot waveguide 12 have opposite width variation trends in the coupling length direction. The width of the strip silicon waveguide 11 gradually decreases as the coupling distance increases, and the width of the slot waveguide 12 gradually increases as the coupling distance increases.

[0044] The slot waveguide 12 includes three layers, specifically including a silicon lower layer 401, a silicon dioxide middle layer 402 as a slot, and a silicon nitride upper layer 403, which are stacked in sequence. At the same time, in order to reduce the photolithography alignment error that may occur between the silicon lower layer 401 and the silicon nitride layer in the actual manufacturing process, the width of the silicon nitride upper layer 403 is greater than the width of the silicon lower layer 401. In addition, the strip silicon waveguide 11 and the slot waveguide 12 may be coated with a silicon dioxide upper cladding layer, and the strip silicon waveguide 11 and the slot waveguide 12 may be filled with a silicon dioxide filling layer, and the silicon dioxide upper cladding layer and the silicon dioxide filling layer are not shown in the figure.

[0045] In this embodiment, the strip silicon waveguide 11 has a first width D1 at the entrance of the tapered coupler 1. Figure 2 As shown, the outlet of the tapered coupler 1 has a second width D2, as shown in FIG. Figure 3 As shown, the first width D1 is greater than the second width D2. The slot waveguide 12 has a third width D3 at the entrance of the tapered coupler 1, as shown in FIG. Figure 2 As shown, the tapered coupler 1 has a fourth width D4 at the outlet, as shown in FIG. Figure 3 As shown, the third width D3 is smaller than the fourth width D4. Specifically, the strip silicon waveguide 11 and the slot waveguide 12 may be tapered, and the two tapered shapes are arranged in opposite directions, wherein the widths of the strip silicon waveguide 11 and the slot waveguide 12 of the tapered coupler 1 change at the same rate, so that the spacing between the strip silicon waveguide 11 and the slot waveguide 12 remains unchanged.

[0046] The strip silicon waveguide 11 and the slot waveguide 12 of the tapered coupler 1 meet the phase matching conditions of the TM polarization state. Compared with the ordinary strip silicon waveguide 11, the silicon nitride / silicon (Si3N4 / Si) slot waveguide 12 can effectively improve the effective refractive index of the TM polarization state, while having little effect on the effective refractive index of the TE polarization state. Therefore, for the TM polarization state, when the strip silicon waveguide 11 and the slot waveguide 12 meet the phase matching conditions, a higher coupling efficiency can be achieved in the tapered coupler 1, while the coupling efficiency of the TE polarization state is significantly suppressed due to the large phase mismatch, so the separation of the TM polarization state and the TE polarization state can be achieved within a shorter distance.

[0047] like Figure 1 and Figure 4 As shown, the polarization filter 2 includes a first polarization filter connected in series to the TE output end of the tapered coupler 1 and a second polarization filter connected in series to the TM output end of the tapered coupler 1 .

[0048] The first polarization filter includes a first strip silicon waveguide 21 and a first slot waveguide 22 for transmitting the TE polarization state and coupling the unnecessary TM polarization state into the first slot waveguide 22 beside the first strip silicon waveguide 21 to couple and filter out the TM polarization state.

[0049] The second polarization filter includes a second strip silicon waveguide 23 and a second slot waveguide 24 , and is used to filter the TE polarization state and convert the polarization mode of the TM polarization state from the second slot waveguide 24 to the second strip silicon waveguide 23 .

[0050] The first slot waveguide 22 and the second slot waveguide 24 each include three layers, specifically including a silicon lower layer 401, a silicon dioxide middle layer 402 as a slot, and a silicon nitride upper layer 403, which are stacked in sequence. At the same time, in order to reduce the photolithography alignment error that may occur between the silicon lower layer 401 and the silicon nitride layer in the actual manufacturing process, the width of the silicon nitride upper layer 403 is greater than the width of the silicon lower layer 401. In addition, the strip silicon waveguide 11 and the slot waveguide 12 may be coated with a silicon dioxide upper cladding layer, and the strip silicon waveguide 11 and the slot waveguide 12 may be filled with a silicon dioxide filling layer, and the silicon dioxide upper cladding layer and the silicon dioxide filling layer are not shown in the figure.

[0051] The polarization filter 2 of the present invention can not only perform the function of polarization filtering, but also realize the function of converting the TM polarization state from the slit waveguide 12 to the conventional silicon waveguide, and the first strip silicon waveguide 21 and the second strip silicon waveguide 23 in the decoupling region of the polarization filter 2 are S-shaped bent to achieve a gradual separation of the waveguide spacing, which is convenient for integration with subsequent other functional devices.

[0052] like Figure 1 As shown, the working principle of the conical polarization beam splitter of this embodiment is as follows:

[0053] The TE / TM polarization state is input from the strip silicon waveguide 11 at the input end of the tapered coupler 1. Since the localization of the TM polarization state in the input strip silicon waveguide 11 is weak, a large part of its mode field is distributed outside the core layer, and the tapered structure of the strip silicon waveguide 11 is expanded from the waveguide width based on the original phase matching condition that satisfies the TM polarization state, so the input TM polarization state has a higher coupling efficiency. However, since the TE polarization state is far away from the phase matching condition, its coupling efficiency is low, and almost no energy will be coupled to the slit waveguide 12. For conventional polarization beam splitters based on linear couplers, the phase matching condition needs to be strictly met in the coupling region to achieve high coupling efficiency, so it is very sensitive to wavelength and device size deviations, and the working bandwidth and process tolerance are limited. The present invention introduces a tapered structure in the coupler so that the coupling of the TM polarization state is no longer very sensitive to wavelength and device size deviations, thereby improving the actual working bandwidth of the device and the ability to tolerate process errors in the actual manufacturing process.

[0054] In order to further improve the extinction ratio, two polarization filters are connected in series at the output port of the tapered coupler 1, and both of them meet the phase matching conditions for the TM polarization state. Among them, the first polarization filter at the TE output port of the tapered coupler 1 can filter out the TM polarization state that is not fully coupled to the slit waveguide 12. And the second polarization filter at the TM output port can couple the TM in the original slit waveguide 12 to the second strip silicon waveguide 23. This filter not only realizes the function of filtering out unnecessary TE polarization states, but also realizes the conversion of TM polarization states from the slit waveguide 12 to the conventional strip silicon waveguide 23, thereby avoiding the introduction of additional mode converters. The device of the present invention can realize the on-chip polarization splitting function, can separate and obtain relatively pure polarization states, and has many potential applications in future polarization multiplexing and sensing.

[0055] like Figure 1 to Figure 5 As shown, this embodiment also provides a method for preparing a tapered polarization beam splitter based on a slit waveguide, the preparation method comprising the steps of:

[0056] Step 1) S11, providing an SOI substrate, and forming the silicon lower layer 401 of all the strip silicon waveguides and the slit waveguides in the SOI substrate by photolithography and etching. In this embodiment, the tapered coupler 1 and the polarization filter 2 of the tapered polarization beam splitter are prepared based on the SOI substrate, and the SOI substrate includes a bottom silicon 301, an insulating layer 302 and a top silicon layer. The lower silicon layers of the strip silicon waveguides and the slit waveguides in this embodiment are prepared based on the top silicon layer. The lower silicon layers of the strip silicon waveguides and the slit waveguides are prepared by etching according to the required patterns of the tapered coupler 1 and the polarization filter 2 described above.

[0057] Step 2) S12, forming the silicon dioxide intermediate layer 402 of the slot waveguide on the silicon lower layer 401. For example, the silicon dioxide intermediate layer 402 of the slot waveguide can be formed on the silicon lower layer 401 by using a PECVD method.

[0058] Step 3) S13, forming the silicon nitride upper layer 403 of the slot waveguide on the silicon dioxide intermediate layer 402. For example, the silicon nitride layer can be deposited by LPCVD method, and the silicon nitride upper layer 403 of the slot waveguide is formed on the silicon dioxide intermediate layer 402 by photolithography and etching process. At the same time, in order to reduce the photolithography alignment error that may occur between the silicon lower layer 401 and the silicon nitride layer in the actual manufacturing process, in this embodiment, the width of the silicon nitride upper layer 403 is greater than the width of the silicon lower layer 401.

[0059] Step 4) S14, forming a silicon dioxide upper cladding layer on the strip silicon waveguide and the slot waveguide.

[0060] As described above, the tapered polarization beam splitter based on the slit waveguide and the preparation method of the present invention have the following beneficial effects:

[0061] In the present invention, by introducing a tapered structure into the coupler, the coupling of the TM polarization state is no longer very sensitive to the wavelength and the size deviation of the device, thereby improving the actual working bandwidth of the device and the ability to tolerate process errors in the actual manufacturing process.

[0062] In order to further improve the extinction ratio, the present invention uses two polarization filters connected in series at the output port of the tapered coupler, and both of them meet the phase matching conditions for the TM polarization state. The polarization filter at the TE output port of the tapered coupler can filter out the TM polarization state that is not fully coupled to the slit waveguide, and the filter at the TM output port can couple the TM in the original slit waveguide to the strip silicon waveguide. The filter not only realizes the function of filtering out the unnecessary TE polarization state, but also realizes the conversion of the TM polarization state from the slit waveguide to the conventional strip silicon waveguide, thereby avoiding the introduction of additional mode converters. The device of the present invention can realize the on-chip polarization splitting function, and can separate and obtain a relatively pure polarization state, and has many potential applications in future polarization multiplexing and sensing.

[0063] The device of the present invention can be prepared by using an integrated process, the process is simple and has a large tolerance, the size is small, a high extinction ratio can be achieved in a wide wavelength range, and integration with other devices is easy to achieve.

[0064] Therefore, the present invention effectively overcomes various shortcomings in the prior art and has high industrial utilization value.

[0065] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A tapered polarization beam splitter based on a slit waveguide, characterized in that: The tapered polarization beam splitter comprises: A tapered coupler and a polarization filter in series; The tapered coupler comprises a strip silicon waveguide and a slot waveguide, wherein the strip silicon waveguide and the slot waveguide have opposite width variation trends in the coupling length direction, wherein the strip silicon waveguide gradually becomes smaller in width as the coupling distance increases, and the slot waveguide gradually becomes larger in width as the coupling distance increases; The polarization filter comprises a first polarization filter connected in series at the TE output end of the tapered coupler and a second polarization filter connected in series at the TM output end of the tapered coupler, wherein the first polarization filter comprises a first strip silicon waveguide and a first slit waveguide, and is used for transmitting the TE polarization state, and coupling the TM polarization state into the first slit waveguide next to the first strip silicon waveguide to couple and filter out the TM polarization state; the second polarization filter comprises a second strip silicon waveguide and a second slit waveguide, and is used for filtering the TE polarization state, and converting the polarization mode of the TM polarization state from the second slit waveguide to the second strip silicon waveguide; The slot waveguide comprises a silicon lower layer, a silicon dioxide middle layer, and a silicon nitride upper layer stacked in sequence; the strip silicon waveguide and the slot waveguide are also covered with a silicon dioxide upper cladding layer.

2. The tapered polarization beam splitter based on a slit waveguide according to claim 1, characterized in that: The width of the silicon nitride upper layer is greater than the width of the silicon lower layer.

3. The tapered polarization beam splitter based on a slit waveguide according to claim 1, characterized in that: A silicon dioxide filling layer is also filled between the strip silicon waveguide and the slot waveguide.

4. The tapered polarization beam splitter based on a slit waveguide according to claim 1, characterized in that: The widths of the strip silicon waveguide and the slot waveguide of the tapered coupler change at the same rate, and the spacing between the strip silicon waveguide and the slot waveguide remains unchanged.

5. The tapered polarization beam splitter based on a slit waveguide according to claim 1, characterized in that: The first strip-shaped silicon waveguide and the second strip-shaped silicon waveguide in the decoupling region of the polarization filter are bent in an S shape.

6. The tapered polarization beam splitter based on a slit waveguide according to claim 1, characterized in that: The tapered coupler and the strip silicon waveguide and the slot waveguide of the polarization filter meet the phase matching condition of the TM polarization state.

7. A method for preparing a tapered polarization beam splitter based on a slit waveguide as claimed in any one of claims 1 to 6, characterized in that: The preparation method comprises the steps of: 1) providing an SOI substrate, and forming the silicon lower layer of the strip silicon waveguide and the slot waveguide in the SOI substrate by photolithography and etching; 2) forming a silicon dioxide intermediate layer of the slot waveguide on the silicon lower layer; 3) forming a silicon nitride upper layer of the slot waveguide on the silicon dioxide intermediate layer; 4) forming a silicon dioxide upper cladding layer on the strip silicon waveguide and the slot waveguide.

8. The method for preparing a tapered polarization beam splitter based on a slit waveguide according to claim 7, characterized in that: Step 2) using the PECVD method to form a silicon dioxide middle layer of the slot waveguide on the silicon lower layer; step 3) using the LPCVD method to deposit a silicon nitride layer, and forming a silicon nitride upper layer of the slot waveguide on the silicon dioxide middle layer through photolithography and etching processes, wherein the width of the silicon nitride upper layer is greater than the width of the silicon lower layer.

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