Grating coupler with reverse T-shaped waveguide structure

Abstract

The invention provides a grating coupler with a reverse T-shaped waveguide structure, which relates to the technical field of photoelectric integrated devices and comprises a photon chip and an optical fiber (1). The photonic chip structure sequentially comprises the following parts from top to bottom: a waveguide coupling grating (2), a waveguide layer (3), a silicon dioxide buried layer (4), a reflector layer (5) and a substrate layer (6). Wherein the reflector layer (5) is formed by a plurality of groups of different material layers with large refractive index differences in an alternating mode, the waveguide coupling grating is formed on the waveguide layer (3) in an etching mode, and the optical fiber (1) is arranged above the photon chip and forms an inclination angle with the z axis in the vertical direction. The reverse T-shaped structure can effectively enhance interaction between adjacent gratings, increase upward diffraction light beams and reduce insertion loss caused by transverse transmission light, so that the coupling efficiency of the grating coupler is improved, and large bandwidth is realized.

Description

Technical Field

[0001] The present invention can be applied to the fields of optical fiber communication, optical interconnection, optical sensing technology, etc., and specifically relates to a grating coupler with an inverted T-waveguide structure, which is a new type of grating coupler with low loss, high coupling efficiency and large bandwidth. Background Art

[0002] In modern communication technology, optical communication has become the main means of information transmission with its significant advantages such as high bandwidth, low loss and strong anti-interference ability. With the exponential growth of Internet data traffic, higher and higher requirements are placed on the performance of optical communication systems. With the continuous advancement of technology, the transmission capacity of optical communication has reached an extremely high level. At present, the transmission capacity of a single optical fiber can reach tens or even hundreds of terabits per second (Tbps). For example, through wavelength division multiplexing (WDM) technology, multiple optical signals of different wavelengths can be transmitted simultaneously in one optical fiber, and each wavelength signal can use a high-order modulation format to further increase the transmission capacity. Optical modules such as 800Gbps and 1.6Tbps have gradually entered the market and have been applied, meeting the current needs for high-speed data transmission in the fields of big data, cloud computing, and artificial intelligence. Therefore, it is urgent to improve the coupling efficiency and bandwidth of optical transmission in the optical communication part.

[0003] Grating couplers based on traditional materials (such as lithium niobate) are limited by the electro-optical coefficient and bandwidth of the material itself when achieving diffraction and coupling. Optical couplers also face challenges in process and material aspects in achieving high-precision, low-loss optical signal distribution. Especially on integrated optical platforms, how to reduce the size of the coupler and improve its performance and alleviate the mutual constraints between coupling efficiency and bandwidth is one of the current research hotspots. Whether from the perspective of meeting the growing demand for optical communications or from the perspective of breaking through the bottleneck of existing technologies and promoting the development of optical communication technology towards higher performance and integration, it has important theoretical and practical significance. Summary of the invention

[0004] In view of this, the present invention provides a grating coupler with an inverted T-type waveguide structure, which can effectively enhance the interaction between adjacent gratings, enhance the upward diffracted light beam, reduce the lateral transmitted light, and reduce the insertion loss, so as to improve the coupling efficiency of the grating coupler.

[0005] According to an embodiment of the present invention, the entire grating coupler structure comprises a photonic chip and an optical fiber (1); the photonic chip comprises, from top to bottom, a waveguide coupling grating (2), a waveguide layer (3), a silicon dioxide buried layer (4), a reflector layer (5), and a substrate layer (6); the optical fiber (1) is arranged above the photonic chip and forms an inclination angle of 3° to 13° with the vertical z-axis;

[0006] According to an embodiment of the present invention, the material of the waveguide coupling grating (2) is set to silicon nitride, and the material of the waveguide layer (3) is set to lithium niobate. The silicon nitride grating waveguide is formed by etching on the lithium niobate film layer. The present invention aims at the existing process problems of the existing lithium niobate grating coupler. The use of silicon nitride to load the lithium niobate waveguide can not only effectively avoid the non-vertical side wall generated during the etching of the lithium niobate waveguide, thereby hindering the realization of the strong coupling waveguide device, but also avoid the undesirable polarization coupling caused by the anisotropy of lithium niobate.

[0007] According to an embodiment of the present invention, the waveguide coupling grating (2) is a uniform symmetrical structure, each period is the same and the range is set to 0.8um to 1.2um.

[0008] According to an embodiment of the present invention, the thickness ranges of the waveguide layer (3) and the waveguide coupling grating (2) are set to 0.1um to 0.4um and 0.25um to 0.4um, respectively. Not only does it meet the needs of single-mode operation, but it also avoids limiting the peak second harmonic generation conversion efficiency due to lateral leakage at the second harmonic wavelength. The waveguide coupling grating (2) is composed of a periodically inverted T-shaped structure produced by two-step etching, and the unetched part of the T-shaped structure is called a long column (21), and its height is consistent with the height of the waveguide coupling grating (2). The shallowly etched part of the T-shaped structure is called a short column (22), and its height ranges from 0.4 to 0.6 times the height of the waveguide coupling grating (2). In theoretical calculations, based on the planar waveguide theory combined with simulation data, it can be concluded that:

[0009] 1) The short column (22) formed by shallow etching in a single period in the waveguide coupling grating (2) has a higher ability to adjust the effective refractive index of the entire grating waveguide than its ability to interact with the light beam. Therefore, in the inverted T-type grating coupler, the short column (22) can be regarded as a factor for controlling the period range and adjusting the effective refractive index of the waveguide, and should be given a shorter width.

[0010] 2) The long column (21) formed by the single-period unetched portion in the waveguide coupling grating (2) has a higher ability to interact with the entire grating waveguide and the light beam than its ability to adjust the effective refractive index. Therefore, in the inverted T-type grating coupler, the long column (21) can be regarded as an important part of the light beam propagation, interaction and diffraction, and its width should be wider than the width of the short column (22).

[0011] 3) Compared with the current two-step etched "L"-shaped structure and "concave"-shaped structure, the inverted T-shaped structure of the waveguide coupling grating (2) is improved in that the gratings of adjacent periods have short columns (22) of the same height, which can better realize the optical coupling between periods, reduce the coupling loss in the light propagation process caused by the height difference of the gratings between periods, and give reasonable parameter values ​​to the long column (21) area of ​​the real light beam propagation to achieve better coupling efficiency and bandwidth.

[0012] According to the above conclusions, the width of the long column (21) ranges from 0.3um to 0.5um, the width of the short column (22) on one side ranges from 0.08um to 0.16um, and the duty ratio of the T-shaped structure ranges from 0.58 to 0.82.

[0013] According to an embodiment of the present invention, the reflector layer (5) is composed of 1 to 5 groups of material layers with a large refractive index difference. In this embodiment, each group is composed of a layer of silicon (51) and a layer of silicon dioxide (52), and the thickness range of each layer is set to 50nm to 200nm. Since the medium with a higher refractive index has a stronger ability to deflect light than the medium with a lower refractive index. The refractive index of the silicon layer (51) is relatively high, about 3.4 to 3.5 (the specific value may vary depending on the wavelength and doping conditions), while the refractive index of the silicon dioxide layer (52) is relatively low, about 1.45 to 1.5. The large refractive index difference can effectively form a bottom reflector. Therefore, when light is emitted from a medium with a high refractive index (such as silicon) to a medium with a low refractive index (such as silicon dioxide), the diffraction grating will form a reflection under non-vertical incidence instead of entering another medium. Therefore, the light beam diffracted downward through the grating structure can be reduced, reducing the loss. Therefore, the thickness of each layer within this range can reflect the downward diffracted light beam to the maximum extent.

[0014] According to an embodiment of the present invention, the input light mode of the waveguide coupling grating (2) is a fundamental mode.

[0015] According to an embodiment of the present invention, the thickness of the silicon dioxide buried layer (4) will affect the coupling efficiency of the grating coupler. The coupling efficiency will produce a wave-like change according to the thickness growth of the silicon dioxide buried layer (4). Therefore, the thickness range is set to 0.5um to 1.5um. This thickness range includes the peak value of the entire change curve and also conforms to the corresponding process range.

[0016] According to an embodiment of the present invention, the optical fiber (1) is arranged 1 um to 10 um above the photonic chip, and forms an inclination angle of 3° to 13° with the vertical z-axis.

[0017] According to an embodiment of the present invention, a grating coupler with an inverted T-type waveguide structure has at least the following beneficial effects:

[0018] 1) The inverted T-type grating coupler provided by the present invention has a bottom reflector layer (5), which reduces the optical power loss leaking to the substrate through the substrate reflection structure, increases the directivity, and improves the coupling efficiency of the grating coupler.

[0019] 2) The novel T-shaped grating waveguide structure provided by the present invention increases the power of the upward diffraction light beam according to the special structure of the long column (21) and the short column (22), reduces the light loss of lateral transmission and the damage to the device caused by the echo light beam generated by lateral transmission.

[0020] 3) The inverted T-type grating coupler provided by the present invention alleviates the mutually restrictive relationship between coupling efficiency and bandwidth, and can achieve a large bandwidth of over 100 nanometers while ensuring coupling efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic structural diagram of a grating coupler of an inverted T-type waveguide structure to which the present invention can be applied;

[0022] Figure 2A This is a diagram of the diffraction field simulation result of the grating coupler of the inverted T-type waveguide structure of the present invention without adding a bottom reflector;

[0023] Figure 2B This is a diagram of the diffraction field simulation result of adding a bottom reflector to the grating coupler of the inverted T-type waveguide structure of the present invention;

[0024] Figure 3A is a diagram of the diffraction field simulation result of the grating coupler based on the inverted T-type waveguide structure of the present invention;

[0025] Figure 3B is a simulation result diagram of the coupling efficiency of the grating coupler based on the inverted T-type waveguide structure of the present invention changing with the wavelength;

[0026] Description of reference numerals:

[0027] 1-optical fiber; 2-waveguide coupling grating; 3-waveguide layer; 4-silicon dioxide buried layer; 5-reflector layer; 6-substrate; 21-long column; 22-short column; 51 silicon layer; 52-silicon dioxide layer. DETAILED DESCRIPTION

[0028] The drawings are for illustrative purposes only and should not be construed as limiting the present patent;

[0029] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0030] Reference Figure 1As shown, the present invention provides a grating coupler with an inverted T-type waveguide structure. It includes a photonic chip and an optical fiber (1). The photonic chip structure includes the following parts from top to bottom: a waveguide coupling grating (2), a waveguide layer (3), a silicon dioxide buried layer (4), a reflector layer (5), and a substrate layer (6); wherein the optical fiber (1) is arranged 1um to 10um above the photonic chip, and forms an inclination angle of 3° to 13° with the vertical z-axis so as to receive light diffracted upward through the grating at an optimal angle. According to an embodiment of the present invention, the material of the waveguide coupling grating (2) is set to silicon nitride, and the material of the waveguide layer (3) is set to lithium niobate. The silicon nitride grating waveguide is formed by etching on the lithium niobate film layer. The light beam is injected from the left side and is transmitted laterally through the waveguide coupling grating (2). In this process, upward and downward diffracted light will be generated. The upward diffracted light beam enters the optical fiber (1). Most of the downward diffracted light is reflected by the reflector layer (5) formed by alternating 1 to 5 groups of silicon layers (51) and silicon dioxide layers (52). A small amount of light will be lost. The inverted T-shaped structure can effectively enhance the interaction between adjacent gratings, increase directivity, reduce insertion loss, and reduce transmitted light, so as to improve the coupling efficiency of the grating coupler and achieve a large bandwidth.

[0031] like Figure 2A and Figure 2B As shown, Figure 2A The internal diffraction field of the grating coupler according to an embodiment of the present invention is shown without adding a bottom reflector; Figure 2B The internal diffraction field of the grating coupler according to the embodiment of the present invention with the bottom reflector (5) added is shown; by comparison, it can be seen that after the bottom reflector (5) is added, most of the light diffracted downward is intercepted and reflected, and the light diffracted upward is equally increased and coupled into the fiber core. Since the reflector (5) is composed of 1 to 5 groups of silicon layers (51) and silicon dioxide layers (52) alternately, a non-vertical light beam will be reflected when it enters a medium with a low refractive index from a medium with a high refractive index. Through simulation calculation and optimization, the thickness of each group of silicon layers (51) and silicon dioxide layers (52) is set to 50nm to 200nm. Furthermore, the distance between the reflector (5) and the grating waveguide is also a major factor affecting the reflection effect. Therefore, the reflector (5) is placed 0.5um to 1.5um below the waveguide layer (3), so the thickness of the silicon dioxide buried layer (4) is set to 0.5um to 1.5um.

[0032] According to an embodiment of the present invention, the single period range of the grating coupler is 0.8um to 1.2um. It can be obtained from the simulation data that, under the same period, different duty cycles can be achieved by simply changing the width of the long column (21) or the width of the short column (22). Both have redshift or blueshift phenomena in the peak wavelength under different duty cycles. In the duty cycle range of 0.58 to 0.82, the coupling curve of only changing the short column (22) at different duty cycles fluctuates around a value and the fluctuation is not large; only changing the long column (21) at different duty cycles gradually improves the coupling efficiency within the duty cycle range. Therefore, only changing the long column (21) has a significantly better effect on the coupling efficiency than the short column (22). Combined with the planar waveguide theoretical formula, it can be inferred that the short column (22) in the inverted T-type grating structure can be regarded as a factor for controlling the period range and adjusting the effective refractive index of the waveguide, and the long column (21) can be regarded as an important part of beam propagation, interaction and diffraction. Based on this conclusion, the long column (21) should be given a wider width. According to the simulation optimization results, the width of the long column (21) is set to 0.3um~0.5um, and the width of the single-sided short column (22) is set to 0.08um~0.16um.

[0033] like Figure 3A and Figure 3B As shown, Figure 3A A diagram showing the diffraction field simulation result after the parameters of the inverted T-type grating coupler are optimized according to an embodiment of the present invention; Figure 3B The simulation result diagram of the coupling efficiency of the inverted T-type grating coupler according to an embodiment of the present invention is shown as a function of wavelength. After the TE fundamental mode passes through the optical waveguide, a strong diffraction effect occurs in the first five periods of the grating, and a large part of the light is diffracted upward through the grating into the fiber core, thereby being coupled into the optical fiber (1). From the coupling curve, it can be seen that the inverted T-type structure is able to maintain a large bandwidth of over 100 nanometers while ensuring high coupling efficiency. This structure can alleviate the mutually restrictive relationship between coupling efficiency and bandwidth, so that the bandwidth can be further expanded under high coupling efficiency, and more information data can be carried, which is of great significance to the further development of optical fiber communication systems.

[0034] Obviously, the above description of the specific embodiments of the present invention is only intended to clearly explain the core concept and application of the present invention, rather than to limit the scope of its application. For those of ordinary skill in the art, based on the above detailed description, other different forms of changes or modifications can also be made. In view of the infinite possibilities of technological development and the diversity of practical application scenarios, it is impossible to list all possible implementation forms one by one in this article. Therefore, it should be clear that any modification, equivalent replacement, and further optimization and improvement made under the guidance of the basic spirit and core principles of the present invention should be deemed to fall within the scope of protection defined by the claims of the present invention.

Claims

1. A grating coupler with an inverted T-type waveguide structure, characterized in that: The entire structure includes a photonic chip and an optical fiber (1); the photonic chip structure includes the following parts from top to bottom: a waveguide coupling grating (2), a waveguide layer (3), a silicon dioxide buried layer (4), a reflector layer (5), and a substrate layer (6); The optical fiber (1) is arranged above the photonic chip and forms an inclined angle with the vertical z-axis.

2. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The waveguide coupling grating (2) is a uniform symmetrical structure, each period is the same and the range is set to 0.8um to 1.2um.

3. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The height range of the waveguide coupling grating (2) is set to 0.25um to 0.4um. The waveguide coupling grating (2) is formed by a periodically inverted T-shaped structure generated by two-step etching. The unetched portion of the T-shaped structure is called a long column (21), and its height is consistent with the height of the waveguide coupling grating (2). The shallowly etched portion of the T-shaped structure is called a short column (22), and its height range is 0.4 to 0.6 times the height of the waveguide coupling grating (2). The width of the long column (21) is in the range of 0.3um to 0.5um, the width of the short column (22) on one side is in the range of 0.08um to 0.16um, and the duty cycle of the T-shaped structure is in the range of 0.58 to 0.

82.

4. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The thickness of the waveguide layer (3) is 0.1um to 0.4um.

5. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The reflector layer (5) is formed alternately by 1 to 5 groups of layers of different materials with large refractive index differences, and the thickness of each layer is set to be in the range of 50 nm to 200 nm.

6. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The input light mode of the waveguide coupling grating (2) is a fundamental mode.

7. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The thickness of the silicon dioxide buried layer (4) is in the range of 0.5um to 1.5um.

8. The grating coupler of the inverted T-type waveguide structure according to claim 1, characterized in that: The optical fiber (1) is placed 1 um to 10 um above the photonic chip, and forms an inclination angle of 3° to 13° with the vertical z-axis.

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