阅读说明：本技术 光栅耦合器及其制作方法、光学相控阵装置 (Grating coupler, manufacturing method thereof and optical phased array device ) 是由 侯昌韬 马丁昽 于 2019-01-30 设计创作，主要内容包括：本发明涉及一种光栅耦合器及其制作方法、光学相控阵装置。该光栅耦合器包括硅基底；硅氧化物层,形成于所述硅基底上；及光栅层,形成于所述硅氧化物层上；其中,所述光栅层包括同层设置的光波导、耦合光栅及反射光栅,所述光波导、反射光栅分别设置于所述光栅层的两端,所述耦合光栅设置于所述光波导和所述反射光栅之间；所述耦合光栅区用于接收所述激光并将所述激光引导至所述光波导,所述反射光栅用于将未被所述耦合光栅引导至所述光波导的激光反射回所述耦合光栅。本申请的光栅耦合器可与CMOS工艺兼容,同时还拥有相对较高的稳定性和相对较小的体积。(The invention relates to a grating coupler, a manufacturing method thereof and an optical phased array device. The grating coupler comprises a silicon substrate; a silicon oxide layer formed on the silicon substrate; and a grating layer formed on the silicon oxide layer; the grating layer comprises an optical waveguide, a coupling grating and a reflection grating which are arranged on the same layer, the optical waveguide and the reflection grating are respectively arranged at two ends of the grating layer, and the coupling grating is arranged between the optical waveguide and the reflection grating; the coupling grating region is used for receiving the laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating. The grating coupler of the application can be compatible with a CMOS process, and meanwhile has relatively high stability and relatively small volume.)

1. A grating coupler for coupling laser light, the grating coupler comprising:

a silicon substrate;

a silicon oxide layer formed on the silicon substrate; and

a grating layer formed on the silicon oxide layer; the grating layer comprises an optical waveguide, a coupling grating and a reflection grating which are arranged on the same layer, the optical waveguide and the reflection grating are respectively arranged at two ends of the grating layer, and the coupling grating is arranged between the optical waveguide and the reflection grating; the coupling grating is used for receiving the laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating.

2. The grating coupler of claim 1, further comprising an index matching layer formed on the grating layer, the index matching layer to reduce reflection of the laser light on the grating layer.

3. The grating coupler of claim 1, wherein the coupling grating is any one of a fully etched grating, a shallow etched grating, a uniform grating, and a binary blazed grating.

4. The grating coupler of claim 1, wherein the reflection grating is a bragg reflection grating.

5. The grating coupler of claim 3, wherein the grating period of the coupled grating is 400 nm to 1000 nm.

6. The grating coupler of claim 4, wherein the grating period of the reflection grating is between 50 nanometers and 600 nanometers.

7. The grating coupler of claim 1, wherein a spacing between the coupling grating and the reflection grating is between 50 nanometers and 500 nanometers.

8. The grating coupler of claim 1, wherein the thickness of the grating layer is 170-270 nm.

9. A method of manufacturing a grating coupler according to any one of claims 1 to 8, the method comprising:

providing a silicon substrate, and depositing and forming a silicon oxide layer on the silicon substrate;

forming a grating film layer on the silicon oxide layer, and etching the grating film layer to form a grating layer; the grating layer comprises an optical waveguide, a coupling grating and a reflection grating, the optical waveguide and the reflection grating are respectively formed at two ends of the grating layer, and the coupling grating is formed between the optical waveguide and the reflection grating; the coupling grating is used for receiving laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating.

10. An optical phased array apparatus, comprising:

a laser source for radiating laser of a preset wavelength;

an optical phase control module for modulating the phase of the laser light; and

an optical coupler for coupling the laser light to the optical phase control module;

wherein the optical coupler is a grating coupler according to any one of claims 1 to 8.

Technical Field

The invention relates to the field of laser detection, in particular to a grating coupler, a manufacturing method thereof and an optical phased array device.

Background

The laser radar is an important sensing device in the field of intelligent driving, and the scanning angle, the scanning stability and the like of the laser radar are required to be improved. Conventional lidar employs an Optical Phased Array (OPA) instead of mechanical scanning to improve the stability of the scanning. When using OPA for emission, it is necessary to couple the laser light from the laser source into the OPA chip for scanning emission. The technology adopted by the OPA chip is CMOS (complementary metal oxide semiconductor) integration technology, the used materials are silicon and silicon oxide, semiconductor lasers and other various lasers cannot use silicon as a substrate material, laser light radiated by the lasers cannot directly enter the OPA chip, and the laser light emitted by the lasers needs to be introduced into the OPA chip through a coupler. The traditional method is to use an optical device such as a lens to couple laser into an OPA chip, but the lens has the problems of large volume, difficult integration and unsuitability for coupling at the chip end. The use of tapered fibers and mode field adapters for coupling results in reduced system stability and increased process complexity.

Disclosure of Invention

Therefore, it is necessary to provide a grating coupler, a method for manufacturing the grating coupler, and an optical phased array device, which are directed to the problems of incompatibility between the existing coupler and the CMOS process, large size, poor stability, and the like.

The application provides a grating coupler for coupling laser, the grating coupler includes:

a silicon substrate;

a silicon oxide layer formed on the silicon substrate; and

a grating layer formed on the silicon oxide layer; the grating layer comprises an optical waveguide, a coupling grating and a reflection grating which are arranged on the same layer, the optical waveguide and the reflection grating are respectively arranged at two ends of the grating layer, and the coupling grating is arranged between the optical waveguide and the reflection grating; the coupling grating is used for receiving the laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating.

In one embodiment, the grating coupler further comprises an index matching layer formed on the grating layer, the index matching layer for reducing reflection of the laser light on the grating layer.

In one embodiment, the coupling grating is any one of a full-etched grating, a shallow-etched grating, a uniform grating and a binary blazed grating.

In one embodiment, the reflective grating is a bragg reflective grating.

In one embodiment, the grating period of the coupling grating is 400 nm-1000 nm.

In one embodiment, the grating period of the reflection grating is 50 nm to 600 nm.

In one embodiment, the coupling grating and the reflection grating are spaced apart by 50 nm to 500 nm.

In one embodiment, the thickness of the grating layer is 170 nm-270 nm.

There is also provided a method for manufacturing a grating coupler, the method being used for manufacturing the aforementioned grating coupler, the method including:

providing a silicon substrate, and depositing and forming a silicon oxide layer on the silicon substrate;

forming a grating film layer on the silicon oxide layer, and etching the grating film layer to form a grating layer; the grating layer comprises an optical waveguide, a coupling grating and a reflection grating, the optical waveguide and the reflection grating are respectively formed at two ends of the grating layer, and the coupling grating is formed between the waveguide and the reflection grating; the coupling grating is used for receiving laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating.

There is also provided an optical phased array apparatus comprising:

a laser source for radiating laser of a preset wavelength;

an optical phase control module for modulating the phase of the laser light; and

an optical coupler for coupling the laser light to the optical phase control module;

the optical coupler is the grating coupler.

According to the grating coupler, silicon is used as a substrate, and then a silicon oxide layer is formed on the substrate, so that the grating coupler can be consistent with a CMOS (complementary metal oxide semiconductor) process of an OPA (optical fiber amplifier) chip, namely can be compatible with the CMOS process; meanwhile, the waveguide and the reflection grating are respectively arranged at two ends of the grating layer, the coupling grating is arranged between the waveguide and the reflection grating, namely the reflection grating is arranged at the back of the coupling grating, so that the laser coupled into the optical waveguide can be emitted from one direction, the laser is prevented from leaking from the back, the coupling efficiency of the laser is improved, and the performance of the grating coupler is further improved; due to the compatibility with CMOS technology, the formed grating coupler has smaller volume than a lens, and the miniaturization and integration of the laser radar are promoted.

Drawings

FIG. 1 is a schematic diagram of a grating coupler according to an embodiment;

FIG. 2 is a perspective view of a grating coupler in one embodiment;

FIG. 3 is a schematic diagram of a grating coupler according to another embodiment;

FIG. 4 is a flow chart illustrating a method for fabricating a grating coupler according to an embodiment;

fig. 5 is a schematic composition diagram of an optical phased array device in an embodiment.

Detailed Description

To facilitate an understanding of the present application, the present application will now be described more fully with reference to the accompanying drawings. Preferred embodiments of the present application are given in the accompanying drawings. This application may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

It will be understood that when an element is referred to as being "secured to" another element, it can be directly on the other element or intervening elements may also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements may also be present. The terms "vertical," "horizontal," "left," "right," and the like as used herein are for illustrative purposes only and do not represent the only embodiments.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the description of the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

Fig. 1 is a schematic structural diagram of a grating coupler according to an embodiment. The grating coupler is used for coupling laser light to an optical waveguide array, and may include a silicon substrate 10, a silicon oxide layer 20, and a grating layer (not labeled in fig. 1). Wherein, a silicon oxide layer 20 is formed on the silicon substrate 10; a grating layer formed on the silicon oxide layer 20; the grating layer comprises an optical waveguide 310, a coupling grating 320 and a reflection grating 330 which are arranged on the same layer, the optical waveguide 310 and the reflection grating 330 are respectively arranged at two ends of the grating layer, and the coupling grating 320 is arranged between the optical waveguide 310 and the reflection grating 330; the coupling grating 320 is for receiving and guiding laser light to the optical waveguide 310, and the reflection grating 330 is for reflecting laser light that is not guided to the optical waveguide 310 by the coupling grating 320 back to the coupling grating 320. The device is realized by adopting a silicon-based material, is compatible with a CMOS (complementary metal oxide semiconductor) process and is lower in cost.

According to the grating coupler, silicon is used as a substrate, and then a silicon oxide layer is formed on the substrate, so that the grating coupler can be consistent with a CMOS (complementary metal oxide semiconductor) process of an OPA (optical fiber amplifier) chip, namely can be compatible with the CMOS process; meanwhile, the waveguide and the reflection grating are respectively arranged at two ends of the grating layer, the coupling grating is arranged between the waveguide and the reflection grating, namely the reflection grating is arranged at the back of the coupling grating, so that the laser coupled into the optical waveguide can be emitted from one direction, the laser is prevented from leaking from the back, the coupling efficiency of the laser is improved, and the performance of the grating coupler is further improved; due to the compatibility with CMOS technology, the formed grating coupler has smaller volume than a lens, and the miniaturization and integration of the laser radar are promoted.

In an embodiment, the silicon substrate 10 may be an insulator made of a silicon material, and the thickness of the silicon substrate 10 may be selected and adjusted according to the actual operation requirement of a person skilled in the art and the performance of the product, and is not further limited herein. The silicon oxide layer 20 may be a silicon dioxide material, it is understood that the silicon oxide layer 20 may also be other suitable compound semiconductor materials, and the thickness of the silicon oxide layer 20 may be selected and adjusted according to the practical operation requirement of a person skilled in the art and the performance of the product, and is not further limited herein. The formation process of the silicon oxide layer 20 may include radio frequency magnetron sputtering, thermal evaporation, vacuum electron beam evaporation, and plasma enhanced chemical vapor deposition process. It is understood that the forming process of the silicon oxide layer 20 can be selected and adjusted according to the actual application and the product performance, and is not further limited herein.

A grating layer (not shown in fig. 1) is formed on the silicon oxide layer 20, and the forming process of the grating layer may include rf magnetron sputtering, thermal evaporation, vacuum electron beam evaporation, and plasma enhanced chemical vapor deposition. It is understood that the forming process of the grating layer can be selected and adjusted according to the actual application and the product performance, and is not further limited herein. The material of the grating layer may be silicon, and it is understood that other materials that can be used for the grating layer may also be selected, which is not limited herein. The thickness of the grating layer can be 170 nm-270 nm, optionally, the thickness of the grating layer is 170 nm-220 nm; optionally, the grating layer has a thickness of 220 nm to 270 nm. In the transmission of the silicon-based optical waveguide, there will generally exist light of different modes, i.e., transverse electric wave TEm and transverse magnetic wave TMm, and the relationship between these two modes and the cut-off thickness of the silicon-based optical waveguide can be calculated by a formula, specifically:

wherein n is_sIs the refractive index of the silicon oxide layer, n_wIs the refractive index of the grating layer, n_cIn order to obtain the refractive index of the dielectric layer formed on the grating layer, n is not formed on the grating layer in the present embodiment_cWhich may be expressed as the refractive index of air, m is an integer, λ is the wavelength of the incident light, L is the cut-off thickness of the grating layer, i.e., the thickness of the grating layer.

In the TE mode, C₁Denotes the TE0 mode, in the TM mode, C₁The TM0 mode is shown so that it can be determined whether the grating layer propagates in the TE0 mode or the TM0 mode at all under different wavelengths of incident light. The thickness of the grating layer can be obtained by formulas a and b, and in this application, the thickness of the grating layer is mainly determined by the transmission mode and the incident angle of the incident light, optionally, the thickness of the grating layer is selected to be 220 nm, and the wavelength of the incident light is selected to be 1550 nm.

Further, please refer to fig. 1, the grating layer includes an optical waveguide 310, a coupling grating region 320 and a reflection grating region 330 that are disposed on the same layer, wherein the optical waveguide 310 and the reflection grating 330 are disposed at two ends of the grating layer, respectively, the coupling grating 320 is disposed between the optical waveguide 310 and the reflection grating 330, in other words, the reflection grating 330 is disposed at the rear of the coupling grating region 320, the coupling grating 320 may be any one of a full-etched grating, a shallow-etched grating, a uniform grating and a binary blazed grating, and in this application, the coupling grating 320 is preferably a full-etched grating. The reflection grating 330 is used to reflect laser light that is not guided to the optical waveguide 310 by the coupling grating 320 back to the coupling grating 320, and the reflection grating 330 may be a bragg reflection grating, it being understood that the reflection grating 330 and the coupling grating 320 in this application may be the same or similar in structure, and at the same time have some similarity in function, which appears to both serve to change the propagation direction of the optical signal. Referring to fig. 1 and 5, when an incident light beam enters the coupling grating 320 in a direction perpendicular to the grating coupler, and is converted by the coupling grating 320, most of the light beam is emitted through the optical waveguide 310 and enters the next device, and a small portion of the light beam enters the reflection grating 330, is reflected by the reflection grating 330, and then returns to the coupling grating 320, and the propagation direction of the light is the same as the propagation direction of the light entering the optical waveguide 310, so that more light beams are absorbed and utilized, thereby improving the conversion efficiency and responsivity of the device.

In an embodiment, please refer to fig. 2, which is a schematic perspective view of a grating coupler in an embodiment. As shown in fig. 2, the geometry of each grating structure 322 in the coupling grating 320 may be a plate, a stripe, or a ridge, and in this application, it is preferable that the grating structures 322 in the coupling grating 320 are stripe structures to perform mode conversion. The geometry of each grating stripe 332 in the reflection grating 330 may also be sheet, stripe or ridge shaped, in this application it is preferred that each grating structure 332 in the reflection grating 330 has the same shape, i.e. stripe structure, as each grating structure 322 in the coupling grating 320. Further, T in fig. 2 denotes a grating period of the coupling grating 320, and Tr denotes a grating period of the reflection grating 330. A transmission grating is further disposed between the coupling grating region 320 and the reflection grating region 330, the transmission grating has a certain width Δ, the width Δ is also the interval between the coupling grating 320 and the reflection grating 330, the width Δ is determined by the reflection coefficient of the grating layer, and in this application, the interval Δ between the coupling grating 320 and the reflection grating 330 is 50 nm to 500 nm. Further, in order to improve the coupling efficiency of the device, in the present application, the grating period T and Tr may be changed, where the grating period T and Tr may be calculated by the following formula:

T×(n_C·sinθ-N_effr)＝mλ(c)

wherein T represents grating period, theta is incident angle, lambda is incident light wavelength, m is integer, N_effrThe effective refractive index of the r-order guided mode, i.e. the effective refractive index of the grating layer. In the present application, since the wavelength of the incident light is selected to be 1550 nm, while at the coupling grating 320, the incident light is incident at an angle perpendicular to the grating layer, the incident angle isTheta is 0 deg., and at the reflection grating 330, the light is incident at an angle of normal incidence, so here the incident angle theta is 90 deg., since m, N_effrIs constant, it is further appreciated that the reflection grating 330 and the coupling grating 320 have different grating periods. In the present application, it should be understood that the coupling grating 320 and the reflection grating 330 have the same etching depth, in other words, since the coupling grating 320 adopts a full etching, that is, the grating layer is completely etched through, the data is used to illustrate that the thickness of the grating layer is 220 nm, the etching depth of the coupling grating 320 is also 220 nm, and correspondingly, the etching depth of the reflection grating 330 is also 220 nm. In this case, the grating period of the coupling grating 320 may be 400 nm to 1000 nm, and optionally, the grating period of the coupling grating 320 may be 400 nm to 750 nm; alternatively, the grating period of the coupling grating 320 may be 750 nanometers to 1000 nanometers; the grating period of the reflection grating 330 may be 50 nm to 600 nm, and optionally, the grating period of the reflection grating 330 may be 50 nm to 350 nm; alternatively, the grating period of the reflection grating 330 may be 350 nm to 600 nm, and it should be understood that the grating period of the reflection grating 330 should be selected to ensure its total reflection capability in the range of 50 nm to 600 nm. In order to further improve the coupling efficiency of the grating and enable the optical signal transmission between the laser and the coupling grating 320 to be more stable and reliable, the present application further sets a duty cycle of the coupling grating 320 and a duty cycle of the reflection grating 330, wherein the duty cycle is defined as a ratio of an etched region to a grating period in each grating period. In the present application, the duty cycle f of the coupling grating 320 is set to 0.8, i.e., f1 equals 0.8, and the duty cycle f2 of the reflection grating 330 equals 0.5. It can be known from the above formula that parameters such as grating period, duty ratio, and etching depth can be controlled to effectively control parameters such as coupling efficiency and spectral width of the grating, so that the transmission of laser in the coupling grating 320 is more stable and reliable.

In one embodiment, please refer to fig. 3, which is a schematic structural diagram of a grating coupler in another embodiment. In the present application, the grating coupler comprises, in addition to the aforementioned structure, a refractive index disposed on a grating layer (not labeled in fig. 3)Matching layer 40, index matching layer 40 is used primarily to reduce reflection of laser light at the grating layer. As described in the preceding examples for a formula, n_cIn order to obtain the refractive index of the medium layer formed on the grating layer, in the present embodiment, since the refractive index matching layer is disposed on the grating layer, n_cMay be expressed as the refractive index of the index matching layer. The surface of the grating layer can be passivated and protected by arranging the refractive index matching layer, so that the comprehensive performance index of the device is improved.

In an embodiment, please refer to fig. 4, which is a flowchart illustrating a method for manufacturing a grating coupler according to an embodiment. The manufacturing method for manufacturing the grating coupler may include steps S10-S20.

In step S10, a silicon substrate is provided and a silicon oxide layer is deposited on the silicon substrate.

Specifically, a silicon oxide layer with a preset thickness can be deposited on the silicon substrate through radio frequency magnetron sputtering, thermal evaporation, vacuum electron beam evaporation and a plasma enhanced chemical vapor deposition process.

Step S20, forming a grating film layer on the silicon oxide layer, and etching the grating film layer to form a grating layer; the grating layer comprises an optical waveguide, a coupling grating and a reflection grating, the optical waveguide and the reflection grating are respectively formed at two ends of the grating layer, and the coupling grating is formed between the optical waveguide and the reflection grating; the coupling grating is used for receiving laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating.

Specifically, a 220 nm grating film layer (not shown) may be formed over a silicon oxide layer by rf magnetron sputtering, thermal evaporation, vacuum electron beam evaporation, and plasma enhanced chemical vapor deposition, and then the grating film layer may be etched by, for example, a dry etching process to form a grating layer, where the grating layer includes an optical waveguide, a coupling grating, and a reflection grating, the optical waveguide and the reflection grating are respectively formed at two ends of the grating layer, and the coupling grating is formed between the optical waveguide and the reflection grating; the coupling grating is used for receiving laser light and guiding the laser light to the optical waveguide, and the reflection grating is used for reflecting the laser light which is not guided to the optical waveguide by the coupling grating back to the coupling grating. In the present application, the etching depth of the coupling grating is the same as the etching depth of the reflection grating, and both are 220 nm. The grating structures of the coupling grating and the reflection grating have the same width and different etching widths so as to ensure that the coupling grating and the reflection grating have different grating periods.

According to the manufacturing method of the grating coupler, the grating coupler is used for manufacturing the grating coupler, and the grating coupler takes silicon as a substrate and then forms a silicon oxide layer on the silicon substrate, so that the grating coupler can be consistent with a CMOS (complementary metal oxide semiconductor) process of an OPA (optical fiber amplifier) chip, namely can be compatible with the CMOS process, and the manufacturing process is simplified; meanwhile, the optical waveguide and the reflection grating are respectively arranged at two ends of the grating layer, the coupling grating is arranged between the optical waveguide and the reflection grating, namely the reflection grating is arranged at the relative rear part of the coupling grating, so that laser coupled into the optical waveguide can be emitted from one direction, the reflection grating can prevent the laser from leaking from the rear part, the coupling efficiency of the laser is increased, and the stability of the grating coupler is further improved; the grating coupler formed has a smaller volume than the lens, since it is compatible with CMOS processes.

Fig. 5 is a schematic structural diagram of an optical phased array device according to an embodiment. As shown in fig. 5, the optical phased array apparatus may include a laser source 1, an optical phased module 3, and an optical coupler 2. The laser source 1 is used for radiating laser with a preset wavelength; in the present application, the laser source 1 is mainly used for radiating laser light of 1550 nm wavelength; the optical phase control module 3 is used for modulating the phase of the laser and simultaneously transmitting the modulated laser at a preset angle after modulation; the optical coupler 2 is used to couple the laser light to the optical phase control module 3, and may be a grating coupler as described above. The grating coupler can be a silicon substrate, a silicon oxide layer and a grating layer, wherein the grating layer can comprise an optical waveguide 310, a coupling grating 320 and a reflection grating 330 which are arranged on the same layer, the optical waveguide 310 and the reflection grating 330 are respectively arranged at two ends of the grating layer, and the coupling grating 320 is arranged between the optical waveguide 310 and the reflection grating 330; the coupling grating 320 is for receiving and guiding laser light to the optical waveguide 310, and the reflection grating 330 is for reflecting laser light that is not guided to the optical waveguide 310 by the coupling grating 320 back to the coupling grating 320.

In the optical phased-array device, the grating coupler is used as the optical coupler, and the grating coupler takes silicon as the substrate and then forms a silicon oxide layer on the silicon substrate, so that the grating coupler can be consistent with a CMOS (complementary metal oxide semiconductor) process of an OPA (optical fiber amplifier) chip, namely can be compatible with the CMOS process; meanwhile, the optical waveguide and the reflection grating are respectively arranged at two ends of the grating layer, the coupling grating is arranged between the optical waveguide and the reflection grating, namely the reflection grating is arranged at the back of the coupling grating area, so that the laser coupled into the optical waveguide can be emitted from one direction, the coupling efficiency of the laser is increased, and the stability of the grating coupler is further improved; the grating coupler formed has a smaller volume than the lens, since it is compatible with CMOS processes.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.