Asymmetric inhomogeneous grating coupler based on lead zirconate titanate film waveguide

Abstract

The present invention provides an asymmetric non-uniform grating coupler based on a lead zirconate titanate thin film waveguide, which relates to the technical field of optoelectronic integrated devices, and comprises: a photonic chip and an optical fiber (8); the photonic chip comprises, from bottom to top, a substrate silicon (1), an oxide buried layer (2), a silicon reflection layer (3), a silicon oxide isolation layer (4), a lead zirconate titanate thin film layer (5), a waveguide coupling grating (6), and a silicon oxide embedding layer (7); the optical fiber (8) is arranged above the silicon oxide embedding layer (7); wherein the waveguide coupling grating (6) comprises, in sequence, a first grating, a second grating and a third grating, which are formed by etching on the lead zirconate titanate thin film layer (5); the first grating, the second grating and the third grating are all asymmetric structures and have different grating periods, so as to reduce back reflection and make the upward diffracted light field of the photonic chip match the mode field of the optical fiber (8).

Description

Technical Field

[0001] The present invention relates to the technical field of optoelectronic integrated devices, and more specifically, to an asymmetric non-uniform grating coupler based on a lead zirconate titanate thin film waveguide. Background Art

[0002] With the rapid development of optical communication technology, the performance requirements for optical interconnect devices are getting higher and higher. Lead zirconate titanate (PZT) has become an ideal material for high-speed electro-optical modulation due to its high electro-optic coefficient and good optical properties. However, the existing PZT grating coupler still has defects in preparation process and performance optimization.

[0003] The efficient coupling between optical fiber and lead zirconate titanate optical waveguide is conducive to further realizing the integrated application of PZT devices. The current grating coupler has the following disadvantages: First, due to the symmetry of PZT, the light will form two symmetrical parts after diffraction by the grating, and part of the optical power will leak to the substrate, resulting in low coupling efficiency of the traditional grating coupler; second, the light field distribution of the traditional uniform period grating coupler diffracts upward exponentially, resulting in low diffraction efficiency of the traditional grating coupler. Summary of the invention

[0004] In view of this, the present invention provides an asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide.

[0005] One aspect of the present invention provides an asymmetric non-uniform grating coupler based on a lead zirconate titanate thin film waveguide, comprising: a photonic chip and an optical fiber; the photonic chip comprises, from bottom to top, a substrate silicon, a buried oxide layer, a silicon reflective layer, a silicon oxide isolation layer, a lead zirconate titanate thin film layer, a waveguide coupling grating, and a silicon oxide embedding layer; the optical fiber is arranged above the silicon oxide embedding layer; wherein the waveguide coupling grating comprises, in sequence, a first grating, a second grating, and a third grating, which are etched on the lead zirconate titanate thin film layer, and the first grating, the second grating, and the third grating are all asymmetric structures and have different grating periods, so as to reduce back reflection and match the upward diffracted light field of the photonic chip with the mode field of the optical fiber.

[0006] According to an embodiment of the present invention, the first grating, the second grating and the third grating are all asymmetric structures. The initial grating length of the first grating is 1-5um, and then the length increases continuously, with an incremental increase of 0.2-0.8um, and continues to increase to the terminal grating of the third grating.

[0007] According to an embodiment of the present invention, the period range of the first grating is 0.8um-1um, and the period range of the second grating and the third grating is 0.8um-0.98um.

[0008] According to an embodiment of the present invention, there is a first partition between the first grating and the second grating, there is a second partition between the second grating and the third grating, and the width of the first partition and the second partition ranges from 0.8um to 1.1um.

[0009] According to an embodiment of the present invention, the duty cycle of the first grating is 0.4-0.5, and the duty cycles of the second grating and the third grating are 0.4-0.6.

[0010] According to an embodiment of the present invention, the etching depths of the first grating, the second grating and the third grating are the same, and the etching depth is 100-200 nm.

[0011] According to an embodiment of the present invention, the optical fiber is arranged 1-10 um above the silica embedding layer and forms an angle of 5°-15° with the vertical direction of the lead zirconate titanate thin film layer.

[0012] According to an embodiment of the present invention, the photonic chip further includes: a conical mode spot changer and a lead zirconate titanate optical waveguide; the conical mode spot changer is arranged between the lead zirconate titanate optical waveguide and the waveguide coupling grating, and is used to perform mode conversion on the light introduced by the lead zirconate titanate optical waveguide and then input it into the waveguide coupling grating.

[0013] According to an embodiment of the present invention, the mode of light input into the waveguide coupling grating is a fundamental mode.

[0014] According to an embodiment of the present invention, the thickness of the lead zirconate titanate thin film layer is 200-400 nm.

[0015] According to an embodiment of the present invention, the thickness of the silicon reflective layer is 0-220 nm.

[0016] An asymmetric non-uniform grating coupler based on a lead zirconate titanate thin film waveguide provided according to an embodiment of the present invention has at least the following beneficial effects:

[0017] (1) The asymmetric non-uniform grating coupler provided by the present invention has a silicon substrate reflection structure. By using an asymmetric structure and adding a silicon substrate reflection structure, the optical power loss leaking to the substrate is reduced, and the coupling efficiency of the grating coupler based on the insulating lead zirconate titanate platform is improved;

[0018] (2) The asymmetric non-uniform grating coupler provided by the present invention optimizes the mode matching between the upward diffraction field and the optical fiber by designing and constructing an asymmetric non-uniform chirped grating coupling structure, and further increases the coupling efficiency of the grating coupler based on the insulating lead zirconate titanate platform;

[0019] (3) The silicon substrate reflective structure of the asymmetric non-uniform grating coupler is consistent with the traditional silicon photonics process flow and is highly compatible with the silicon-on-insulator process, which is conducive to promoting the development of silicon-based lead zirconate titanate heterogeneous integration process technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0021] Figure 1 The schematic diagram of the structure of the asymmetric non-uniform grating coupler based on the lead zirconate titanate thin film waveguide to which the present invention can be applied is shown;

[0022] Figure 2 A schematic top view of an asymmetric non-uniform grating coupler used in conjunction with a tapered pattern spot changer according to an embodiment of the present invention is shown;

[0023] Figure 3 A diagram schematically shows a simulation result of the internal diffraction field of an asymmetric non-uniform grating coupler according to an embodiment of the present invention;

[0024] Figure 4 A diagram schematically shows the simulation result of the effect of the length Lt of the tapered mode spot changer on the fundamental mode conversion efficiency according to an embodiment of the present invention;

[0025] Figure 5 A diagram schematically shows a simulation result of the coupling efficiency of an asymmetric non-uniform grating coupler varying with wavelength according to an embodiment of the present invention;

[0026] Fig. 6A The schematic diagram shows the internal diffraction field of the asymmetric non-uniform grating coupler without a silicon reflective layer according to an embodiment of the present invention;

[0027] Figure 6B The figure schematically shows the internal diffraction field of the asymmetric non-uniform grating coupler according to the embodiment of the present invention when there is a silicon reflective layer.

[0028] Description of reference numerals:

[0029] 1-silicon substrate; 2-buried oxide layer; 3-silicon reflective layer; 4-silicon oxide isolation layer; 5-lead zirconate titanate thin film layer; 6-waveguide coupling grating; 7-silicon oxide embedding layer; 8-optical fiber; 9-conical mode spot changer; 10-lead zirconate titanate optical waveguide. DETAILED DESCRIPTION

[0030] Below, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of concepts of the present invention.

[0031] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0032] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0033] When using expressions such as "at least one of A, B, and C, etc.", they should generally be interpreted according to the meaning of the expression commonly understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0034] like Figure 1 As shown, the present invention provides an asymmetric non-uniform grating coupler based on a lead zirconate titanate thin film waveguide, comprising: a photonic chip and an optical fiber 8; the photonic chip comprises, from bottom to top, a substrate silicon 1, an oxide buried layer 2, a silicon reflective layer 3, a silicon oxide isolation layer 4, a lead zirconate titanate thin film layer 5, a waveguide coupling grating 6, and a silicon oxide embedding layer 7; the optical fiber 8 is arranged above the silicon oxide embedding layer 7; wherein the waveguide coupling grating 6 comprises, in sequence, a first grating, a second grating, and a third grating, which are formed by etching on the lead zirconate titanate thin film layer 5. The first grating, the second grating, and the third grating are all asymmetric structures and have different grating periods. Through the light diffraction effect of the waveguide coupling grating 6, the light from the lead zirconate titanate optical waveguide 10 is coupled into the optical fiber 8 to achieve mode field matching between the field and the optical fiber 8.

[0035] like Figure 2As shown, the photonic chip of the present invention further includes a conical pattern changer 9 and a lead zirconate titanate optical waveguide 10. The conical pattern changer 9 is arranged between the lead zirconate titanate optical waveguide 10 and the waveguide coupling grating 6. Since the width difference between the lead zirconate titanate optical waveguide 10 and the waveguide coupling grating 6 is large, if the lead zirconate titanate optical waveguide 10 is directly connected to the waveguide coupling grating 6, a large mode mismatch will be generated. Therefore, a conical pattern changer 9 is used to convert the mode of the light introduced by the lead zirconate titanate optical waveguide 10 and then input it into the waveguide coupling grating 6 to achieve fundamental mode conversion between the two.

[0036] like Figure 1 As shown, the optical fiber 8 is arranged 1-10 um above the silicon oxide embedding layer 7 and forms an angle of 5° to 15° with the vertical direction of the lead zirconate titanate film layer 5. Through software simulation, it can be obtained that the best placement position of the optical fiber 8 is 2 um above the silicon oxide embedding layer 7.

[0037] The first grating, the second grating and the third grating are asymmetric uniform gratings. Figure 1 As shown in FIG. 1 , from left to right are the first grating P1, the second grating P2 and the third grating P3. Figure 2 As shown, the initial grating length of the first grating is L1, which is 1-5um, and then the length continues to increase, with a length increment of a=0.2-0.8um, and continues to increase to the terminal grating of the third grating. After simulation optimization, the optimal values ​​are L1=4um and a=0.5um. The design of the asymmetric structure can reduce back reflection.

[0038] The first grating P1 is a long-period grating, and its period range can be 0.8um-1um. The second grating P2 and the third grating P3 are short-period gratings, and their period range can be 0.8um-0.98um. After simulation optimization, the optimal period of the first grating P1 is 0.915um; the optimal period of the second grating P2 and the third grating P3 is P2=0.885um, P3=0.85um. By optimizing the grating period, the mismatch between the upward diffraction field and the internal mode field of the optical fiber is improved, and the coupling efficiency is improved.

[0039] There is a first partition gap1 between the first grating P1 and the second grating P2, and a second partition gap2 between the second grating P2 and the third grating P3. The width range of the first partition and the second partition is 0.8um-1.1um. Through simulation, it can be seen that the change of the first partition gap1 and the second partition gap2 will have a greater impact on the coupling efficiency. The optimal values ​​after simulation are gap1=0.85um and gap2=1.1um.

[0040] The duty cycle D1 of the first grating P1 can be 0.4-0.5, and the duty cycles D2 and D3 of the second grating P2 and the third grating P3 can be 0.4-0.6. After simulation optimization, the optimal values ​​are D1=0.48 and D2=D3=0.6 respectively.

[0041] The etching depths of the first grating P1, the second grating P2 and the third grating P3 are the same, and are 100-200 nm.

[0042] The main disadvantages of standard uniform grating couplers are the narrow bandwidth (typically between 30-40 nm, less than half of edge-coupled solutions) and the relatively low coupling efficiency (CE). The relatively low CE of grating couplers can be attributed to two main factors: grating directivity and mode mismatch. Considering the beam coupling from the waveguide to the fiber, the low grating directivity (≈65%) means that a large part of the optical power incident on the grating coupler is diffracted to the substrate (≈35%) instead of being sent to the fiber. Mode mismatch limitation is due to the fact that the light intensity distribution radiated by the uniform grating exhibits a nearly exponential decay shape, with a reduced overlap integral with the mode distribution of the standard fiber (nearly Gaussian). The directivity can be improved by reducing the power leaking to the substrate. Fresnel reflection can be described as the reflection that occurs when light propagates between media with different refractive indices. For grating couplers, the effective refractive indices of the grating region (Bloch modes) and the slab region can describe these different effective media. Therefore, Fresnel reflection occurs on grating couplers.

[0043] In the disclosed embodiment, in order to reduce Fresnel reflection in the grating coupler, the grating grooves will be etched asymmetrically. Therefore, the effective refractive index of the Bloch mode on the grating coupler is asymmetric. The light from the entrance waveguide will be reflected by the asymmetric boundary, and the reflection direction will change. Therefore, the back reflection will be reduced. By using a non-uniform grating, the energy distribution of the upward diffraction field is made Gaussian, which can overcome the mode mismatch between the Gaussian distribution of the fiber mode field and the exponential decay mode field of the uniform grating.

[0044] The thickness of the lead zirconate titanate thin film layer 5 is 200-400nm. Existing grating couplers often use lithium niobate as a preparation material. As an electro-optical material, lithium niobate has attracted much attention due to its Pockels effect, especially in achieving high-bandwidth and high-linearity electro-optical modulation. Many research institutions at home and abroad are actively studying thin-film lithium niobate Mach-Zehnder electro-optical modulators (MZMs), which have been able to achieve a modulation bandwidth of more than 70GHz, an optical loss of less than 1dB / cm, and a high modulation efficiency of 22V·mm. Using PAM4 modulation technology, it can support a single-channel transmission rate of up to 200Gbps. However, the electro-optic coefficient of lithium niobate material is relatively low (r33=31 pm / V), which limits the possibility of further reducing the driving voltage and device size. In addition, lithium niobate has poor compatibility with silicon and needs to rely on complex wafer bonding processes, which affects its cost-effectiveness. In comparison, lead zirconate titanate (Pb(Zr 0.52 Ti 0.48 )O3, PZT) is a ferroelectric oxide that has attracted widespread attention due to its high transparency and higher Pockels coefficient (80~130pm / V). Studies have shown that when the titanium element in PZT accounts for 48%, the material is at a quasi-modular phase boundary, at which time the electro-optic coefficient reaches the optimal level, showing excellent electro-optical properties, piezoelectric properties and fatigue resistance. More importantly, PZT can achieve large-scale, high-quality crystal film epitaxial growth on a silicon substrate through wet deposition technology, and the film thickness is controllable to meet the needs of different application scenarios. This makes it possible to realize a CMOS-compatible high-performance silicon-based / PZT heterogeneous integrated optical platform, which is expected to solve the problem of low-driving voltage and high-speed electro-optical modulation devices on a silicon-based platform.

[0045] The thickness of the silicon reflective layer 3 can be 0-220 nm. By adding a silicon substrate reflective structure and an asymmetric structure, the optical power loss leaking to the substrate is reduced, and the coupling efficiency of the grating coupler based on the insulating lead zirconate titanate platform is improved.

[0046] In one embodiment of the present invention, the etching depth of the first grating P1, the second grating P2 and the third grating P3 can be 150nm, the thickness of the silicon oxide isolation layer 4 can be 1um, the thickness of the silicon reflective layer 3 is 220nm, the thickness of the silicon oxide buried oxide layer 2 is 2um, and the thickness of the silicon substrate 1 is set to 500um.

[0047] like Figure 2 As shown, the length of the conical mode spot changer 9 connected to the waveguide coupling grating 6 is set to Lt, and its function is to realize the mode conversion between the lead zirconate titanate optical waveguide 10 and the waveguide coupling grating 6, and to ensure that the mode entering the grating area is the fundamental mode as much as possible.

[0048] like Figure 3As shown, after the TM fundamental mode passes through the lead zirconate titanate optical waveguide 10, the diffraction field diagram inside the asymmetric non-uniform grating coupler is obtained. It can be found that when the TM fundamental mode from the lead zirconate titanate optical waveguide 10 passes through the grating area, a strong diffraction effect will occur, and a large part of the light enters the fiber core through the upward diffraction of the grating, and is coupled into the optical fiber. Among them, the x direction represents the direction of light transmission from the lead zirconate titanate optical waveguide 10 to the grating area, and the y direction represents the direction from the grating to the optical fiber.

[0049] like Figure 4 As shown, the influence of the length Lt of the conical mode spot changer 9 on the fundamental mode conversion efficiency was investigated through simulation, and the optimal value of Lt was obtained to be 70um.

[0050] like Figure 5 As shown, in the embodiment of the present invention, by optimizing and simulating parameters such as the grating period, the grating duty cycle, and the separation gap, a higher coupling efficiency is theoretically obtained.

[0051] like Fig. 6A and Figure 6B As shown, Fig. 6A The schematic diagram shows the internal diffraction field of the asymmetric non-uniform grating coupler without a silicon reflective layer according to an embodiment of the present invention; Figure 6B The internal diffraction field of the asymmetric non-uniform grating coupler according to an embodiment of the present invention when there is a silicon reflective layer is schematically shown. The internal diffraction field of the asymmetric non-uniform grating coupler with and without the silicon reflective layer is compared by simulation. It can be seen that when the silicon reflective layer is added, most of the light enters the fiber core through the upward diffraction of the grating.

[0052] The embodiments of the present invention are described above. However, these embodiments are only for the purpose of illustration, and are not intended to limit the scope of the present invention. Although each embodiment is described above, it does not mean that the measures in each embodiment cannot be used in combination advantageously. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which should all fall within the scope of the present invention.

Claims

1. An asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide, It is characterized in that include: Photonic chips and optical fibers (8); The photonic chip comprises, from bottom to top, a silicon substrate (1), a buried oxide layer (2), a silicon reflective layer (3), a silicon oxide isolation layer (4), a lead zirconate titanate thin film layer (5), a waveguide coupling grating (6), and a silicon oxide buried layer (7); The optical fiber (8) is arranged above the silicon oxide embedding layer (7); The waveguide coupling grating (6) comprises a first grating, a second grating and a third grating in sequence, which are formed by etching on the lead zirconate titanate thin film layer (5), and the first grating, the second grating and the third grating are all asymmetric structures with different grating periods, so as to reduce back reflection and make the upward diffracted light field of the photonic chip match the mode field of the optical fiber (8); There is a first partition between the first grating and the second grating, there is a second partition between the second grating and the third grating, and the width of the first partition and the second partition ranges from 0.8um to 1.1um; The photonic chip further comprises: A conical spot mode changer (9) and a lead zirconate titanate optical waveguide (10); The conical pattern changer (9) is arranged between the lead zirconate titanate optical waveguide (10) and the waveguide coupling grating (6) and is used to perform mode conversion on the light introduced by the lead zirconate titanate optical waveguide (10) and then input the light into the waveguide coupling grating (6).

2. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The first grating, the second grating and the third grating are all asymmetric structures; The grating lengths of the first grating, the second grating and the third grating are increased in sequence. The initial grating length of the first grating is 1-5 um, and the grating increment is 0.2-0.8 um.

3. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The period range of the first grating is 0.8um-1um, and the period range of the second grating and the third grating is 0.8um-0.98um; the duty cycle of the first grating is 0.4-0.5, and the duty cycle of the second grating and the third grating is 0.4-0.

6.

4. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The etching depths of the first grating, the second grating and the third grating are the same, and the etching depth is 100-200 nm.

5. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The optical fiber (8) is arranged 1-10 um above the silicon oxide embedding layer (7) and forms an angle of 5°-15° with the vertical direction of the lead zirconate titanate thin film layer (5).

6. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The mode of light input into the waveguide coupling grating (6) is the fundamental mode.

7. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The thickness of the lead zirconate titanate film layer (5) is 200-400 nm.

8. The asymmetric non-uniform grating coupler based on lead zirconate titanate thin film waveguide according to claim 1, It is characterized in that The thickness of the silicon reflective layer (3) is 0-220 nm.

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