Review of the technology of a single mode fiber coupling to a laser diode

Optical Fiber Technology 55 (2020) 102097

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Review of the technology of a single mode fiber coupling to a laser diode Haibo Zhou, Hongwei Xu, Ji-an Duan

⁎

T

State Key Laboratory of High Performance Complex Manufacturing, College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China

ARTICLE INFO

ABSTRACT

Keywords: Single mode fiber Laser diode Bulk optics Microlens fiber Coupling efficiency Optical structure Manufacturing technology

In this paper, the technology of a single mode fiber coupling to a semiconductor laser diode has been summarized and the latest developments in the bulk optics coupling scheme and the microlens fiber coupling scheme have been reviewed. The review has focused on optimizing the optical structure and the coupling parameters to improve the coupling efficiency and packaging performance. The advanced manufacturing technology as well as common modeling methods and applications of coupling systems have also been reviewed. Finally, the paper has summarized the key technologies of a single mode fiber coupling to a laser diode and its direction of development in the future.

1. Introduction High-speed information has become the key to communication technologies with the rapid development of information technology. Ever since it was first proposed [1], Fiber-optic communication [2,3], as an important revolution in the history of modern communication, has been rapidly developed and has many excellent properties, such as large transmission capacity, long transmission distance, strong anti-interference ability, good transmission quality, good confidentiality, etc. The main component of the optical fiber communication system has been shown in Fig. 1. In order to improve the transmission capability and the fidelity, it is necessary to reduce the transmission loss of the optical fiber, and improve the coupling efficiency between the light source and the optical fiber. With the improvement of optical fiber manufacturing technology and the emergence of new types of optical fibers, the transmission loss of optical fibers has dropped to a very low level [4,5]. Meanwhile, semiconductor lasers have become the most reliable and fastest-growing fiber-optic communication light source; they have many advantages, such as small size, light weight, simple structure, etc. The coupling efficiency between the semiconductor laser and the fiber as a key parameter characterize the performance and the reliability of optical fiber communication systems. A lot of work has been carried out to investigate the fiber pigtailed laser diode and its applications in optical networks, such as data switching [6], ring resonance [7], WDM-PON transmission [8] and optical injection locking [9]. The problem of coupling loss between the light source and the optical fiber has become more and more prominent. Thus, it is of great importance to study laser coupled transmission technology and provide a theoretical basis for optical signal transmission and receiver module ⁎

packaging. In recent years, the research on optical fiber coupling systems at home and abroad has mainly focused on the development of new optical lenses to improve the optical coupling performance of the system. The effects of various parameters on the coupling efficiency are studied by establishing a more efficient coupling model of an optical system. A high coupling efficiency and good process parameters can be ensured by optimizing the structure parameters and the coupling parameters. In previous studies, various optical coupling techniques have been used to achieve a higher coupling efficiency. Generally, the spot from the light source is adjusted by a special optical structure, thereby increasing the energy of the laser that enters the optical fiber. This paper has summarized the technology of a single mode fiber coupling to a semiconductor laser diode and has reviewed the latest developments in the bulk optics coupling scheme and the microlens fiber coupling scheme. The review focused on optimizing the optical structure and the coupling parameters to improve the coupling efficiency and packaging performance. The advanced manufacturing technology, as well as common modeling methods and applications of coupling systems, have also been reviewed. The bulk optics coupling scheme and the microlens fiber coupling scheme have also been covered. 2. Optical basis A deterministic optical coupling system is a system whose main parameters include internal factors (laser wavelength and beam waist radius [10], lens shape and refractive index, fiber diameter and refractive index) and external factors [11] (lateral alignment error,

Corresponding author. E-mail address: [email protected] (J.-a. Duan).

https://doi.org/10.1016/j.yofte.2019.102097 Received 4 September 2019; Received in revised form 18 November 2019; Accepted 22 November 2019 1068-5200/ © 2019 Published by Elsevier Inc.

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Fig. 1. The main component of the optical fiber communication system.

longitudinal alignment error and rotational angle alignment error) [12]. In order to optimize the coupling system, it is necessary to establish a coupling model. The first thing to do is to find expressions that describe the relevant physical quantities. A simple technique has been proposed to evaluate the modal spot size of a trapezoidal index fiber [13]: by means of the quadratic polynomial fitting, the focusing constant of a gradient-index fiber was measured and analyzed [14]. The validity of the approximation, the fundamental mode of a graded-index fiber [15] which can be approximated by a Gaussian function, was then verified, and it is particularly good for the parabolic index fiber [16]. Two variational approximations of the fundamental mode of a single mode fiber have been presented, which are more accurate when compared with the Gaussian approximation [17]. Generally, based on the Gaussian approximation, the modal field of the light source and a single mode fiber, which have been shown in Fig. 2, can be expressed by an elliptical Gaussian beam and a Circular Gaussian beam, respectively:

Ef = Exp

x 2 + y2 2 f

, Eld = Exp

x2 2 ex

+

y2 2 ex

ik

coupling surface:

=

|Eld

Eld Ef |2 dxdy

dxdy 2 |Ef |2 dxdy

.

(2)

The theoretical maximum coupling efficiency can be obtained by optimizing the parameters and minimizing the error. A superior optical coupling system should also be easy to process and package. It is desirable to have a large tolerance and a large operating space in the case of high coupling efficiency [11,21]. High coupling efficiency and good process parameters can be ensured by optimizing the structure parameters and coupling parameters. 3. Optical structure One or more optical elements are added between the fiber and the light source to shape the spot, so that the energy entering the end face of the fiber is as high as possible. According to the type of optical structure, the coupling modes can be divided into bulk optics coupling and microlens fiber coupling. The basic schematics of them have been shown in Fig. 2.

y2 x2 + 2R ex 2R ey

3.1. Bulk lens coupling scheme

(1) where f is the mode field radius of the single mode fiber, ex and ey are the beam waist radii perpendicular and parallel to the junction plane of the laser diode facet respectively, R ex and R ey are the radii of the curvature of the wave front from the laser diode perpendicular and parallel to the junction plane respectively, k is the wavenumber of the incident medium. As shown in Fig. 2, there is a mismatch between the output mode field of the semiconductor laser and the intrinsic mode field of the fiber. For research needs, a coupling surface is selected on one side of the optical structure, close to the fiber or close to the semiconductor laser. The coupling efficiency can be obtained by performing an overlap integration [18–20] on the two light fields that are to be coupled on the

The bulk lens coupling scheme refers to one or more bulk lenses that are added between the fiber and the light source. Generally these are separate micro lens couplings, including ball lens couplings, cylindrical lens couplings, graded index lens couplings, combined lens couplings, etc. The main structural forms have been shown in Fig. 3. Due to spherical aberrations [22], a limited aperture [23] and a mode mismatch, the ball lens produces a lower excitation efficiency [10]. The ball lens coupling is still widely used in optical fiber coupling systems because of its circular symmetry, convenient packaging, simple manufacturing and low cost. To minimize the spherical aberration, an effective way can be to place the laser diode closer to the ball lens, and use a higher refractive index and a smaller diameter for the ball lens

Fig. 2. Bulk optics coupling and microlens fiber coupling. 2

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between fibers with a tapered hyperbolic microlens and a tapered hemispherical microlens, and a much higher coupling efficiency was achieved in the tapered hyperbolic microlens [37]. An ABCD matrix was used to investigate the coupling efficiency of a hyperbolic microlens fiber for two different wavelengths, namely 1.3 and 1.5 μm, and a higher efficiency was obtained at 1.3 μm [38]. A cylindrical lensed fiber [39] and an elliptic parabolic lensed fiber [40] has been investigated using the ABCD matrix as well. Based on scalar diffraction and Gaussian mode shapes, a double wedge microlens was investigated to improve the coupling efficiency [41]. The coupling efficiency between a laser diode and a single mode fiber was maximized using a novel fiber with an upside down tapered microlens on the fiber tip [42]. With the help of the thermally expanded core technique, a high numerical aperture thermally expanded core fiber was proposed to improve the coupling efficiency [43,44]. The thermally expanded core fiber scheme has a higher coupling efficiency and larger lateral and longitudinal tolerances, but it has a lower tilt tolerance when compared to the conventional single mode fiber scheme. A wedge shaped single mode fiber was investigated by the beam propagation method, and a much higher coupling efficiency was achieved by a vertical cut at the fiber tip [45]. A quadrangular-pyramid-shaped fiber end-face [46] and a conical-wedgeshaped fiber end-face [47] was used to control two axial curvatures to better match the far field of the laser diode. Fibers with different kinds of core and index, as well as different kinds of microlens on the fiber tip were comprehensively investigated to improve the coupling efficiency between the laser diode and the microlens fiber; the details of which have been shown in Table 1. Among the commonly used single mode fiber, including circular core and elliptic core, the former is more widely used. From the use of different refractive index fibers, it can be seen that step-index fibers have the same application potential as graded-index fibers. The shape of the fiber tip is mainly hemispherical, parabolic, and hyperbolic. Different coupling effects can be obtained by combining different fiber cores, different fiber refractive indices, and different shapes of the lens tip. Differing from Table 1, Table 2 has shown two optical structures that were used simultaneously to focus the light beam from the laser diode to a single mode fiber. As can be seen from the above two tables, the researchers realized optical path shaping by combining different optical structural elements, so as to explore the best coupling scheme. New methods were also used in the research. A finite difference time domain method was used to analyze a tapered-cladding tapered lens fiber and a tapered-core tapered lens fiber; of these the tapered-core tapered lens fiber has a smaller coupling loss [79,80]. The coupling efficiency of a hemispherical lens was investigated by the method of lines, which has a higher precision when compared to the ABCD matrix [81]. The focusing constant of a gradient index fiber lens was measured using the curvefitting algorithm; then the gradient index fiber probes was designed by the researchers [14].

Fig. 3. Diagram of commonly used bulk optics coupling schemes.

[24]. The ball lens coupling scheme is the coupling scheme that is least sensitive to misalignment when compared to a conic lens and a graded index lens [25]. That is to say, the ball lens coupling scheme has lower requirements for its packaging equipment. A performance comparison was made between ball lenses that had a gradient index and a homogeneous medium [26]; the results showed that the former is superior to the latter. A single biconvex microlens was presented for focusing the laser diode beam as well as for correcting the astigmatism and ellipticity [27]. There are three profiles on both the input and output surfaces, which are fabricated by the excimer laser dragging method. The position deviations of the coupling system, which are the offaxis deviation, the defocus deviation, and the angle deviation, have a great affect on the coupling efficiency. Two ball lenses of the same diameter and different refractive indices were used to make the coupling more efficient and the misalignment tolerances more relaxed [28]. The tolerance ranges of the factors mentioned above were evaluated quantitatively, as a strict coupling loss limit of less than 1 dB was demanded [12]. It is difficult to precisely adjust the fiber’s position using a single lens. It has been proved that a combination lens coupling can improve the coupling efficiency [29]. A good way to achieve auto-focusing and auto-tracking, which provides high accuracy and large dynamic range, was to use two cylindrical lenses, which allowed the high accuracy and a large dynamic range to be maintained simultaneously [30]. A method, with a planoconvex cylindrical lens and an asphere lens, for beam shaping design has been presented in the literature [31]. The shaping of the beam was then divided into two steps: first, the light beam was transformed from an oval spot into a circular spot, and then it was collimated. The fundamental mode of single mode elliptic core fibers was analyzed by Gaussian approximation, and the longitudinal, transverse and azimuthal expressions were obtained [32]. The optical simulation software Zemax [33,34] has been used to optimize the microlens [35,36]. All in all, to increase the coupling efficiency and raise the process tolerance of an external bulk lens’ coupling scheme, a better way could be to reduce the coupling loss caused by physical effects, choose a suitable material and explore new structures for the bulk lens.

4. Manufacturing technique

3.2. Microlens coupling scheme

It has been found that, although a tapered hyperbolic-shaped fiber can theoretically reach up to a 100% coupling efficiency [48,82,83], it is difficult and costly to fabricate. The processing performance of an optical structure is also an important index for evaluating the quality of it. Advanced manufacturing technology ensures a high coupling efficiency. Generally, the manufacturing methods include a physical method and a physical–chemical method.

The microlens coupling scheme refers to either directly processing or attaching a particular microlens to the end-face of a single mode fiber. Commonly, the shape of the microlens can be hemispherical, hyperbolic, parabolic, etc. Recently much attention has been paid to the application of the microlens fiber, which forms the lens structure on the fiber end directly, for optical fiber communication systems. This can be attributed to their simple and compact structure, one-step alignment with the laser diode, their simple fabrication, and the low production cost. Several studies of lensed fibers have been reported, the following mainly refers to the structure and manufacturing technique of an intrinsic microlens. Similar to the external bulk lens coupling scheme, the structures of a microlens are crucial for coupling efficiency. A comparison was made

4.1. Physical method The physical method is mainly used to directly remove the material using an external force to obtain a specified shape. There is also a way to obtain the specified shape by means of hot melt cooling. A direct laser processing technique, which has great flexibility with 3

Optical Fiber Technology 55 (2020) 102097

·

Middle

Fiber

Ref

Gradient index fiber Parabolic index fiber Hemispherical lenses Coreless hyperbolic lenses

No-core fiber Silica fiber Long period grating Graded index ovalcore fiber Gradient index fiber Gradient index fiber

Single mode fiber

[14] [75] [76] [77]

·

Gradient index cylindrical lenses Gradient index hemispherical lenses

· · · ·

· · ·

· ·

·

·

·

·

·

·

·

· ·

Upside down tapered

Quadric interface Microlens

Hemispherical Parabolic Hyperbolic Hemispherical Hyperbolic

· Step index Graded index Fiber index

Triangular Parabolic Trapezoidal Elliptical core Circular core Fiber core

[78] [92]

regard to lens size and radius curvature, has been used to directly fabricate a microlens upon the end-face of a single mode fiber [84]. However the surface quality of a microlens fabricated using this method should be further improved upon. The excimer laser biaxial dragging method, which is suitable for fabricating different profiles in one surface, has been used to fabricate a biconvex microlens [27,36] and a plano-convex aspherical microlens [35], together with a bi-axial contour mask scanning method. Based on the elastic polishing plate mechanism, a lensed fiber fabrication workstation has been presented in the literature to fabricate high precision lens geometry [85]. An up-tapered structure was incorporated to decrease transverse misalignment [86]. A single-step grinding technique, together with fine polishing by arc heating, was used to fabricate a double variable curvature microlens [87], and an asymmetric elliptical cone shaped microlens [88]. Grinding and polishing techniques were used to fabricate a quadrangular-pyramidshaped fiber end-face [46], and a conical-wedge-shaped fiber end-face [47]. A CO2 laser was used to fabricate a microlens on the end-face of a single mode fiber [83,89]. The performance, consistency, and speed of this method are better than the etch-and-melt technique [90,91]. A fiber with a tapered hemispherical end was drawn by arc discharge [92–94]. The fabrication of a novel structure of a lensed plastic optical fiber, which was fabricated using electrostatic force, has been demonstrated and the coupling efficiency exceeded 72% [95]. A focused ion beam [96,97] and thermal reflow [98,99] was used to fabricate a micro-lens. An electrostatic pulling method was used to fabricate a cone shaped microlens [100]. Grinding and fusing techniques were used to fabricate a hyperboloid microlens, a coupling efficiency of 83% was achieved [101]. A scheme was proposed to improve the coupling efficiency between the EDF and the SMF using a tapered fiber tip, which was obtained by fusing the erbium-doped fiber with a tipped singlemode fiber with a lensed end-face [102]. As a result, the quantum efficiency of the laser was improved by nearly two times the original value. The laser direct writing technique [103], which has been shown in Fig. 4, was used to fabricate a microlens on the tip of a single mode fiber, and a coupling efficiency of 53.5% was achieved at a working distance of 16 μm. Grinding, spin-on-glass coating and the electrostatic pulling process were used to fabricate a hyperbolic microlens [104,105] on a fiber tip, the fabrication process has been shown in Fig. 5. The coupling efficiency was higher than 75% [105].

·

· · · · · · · · ·

·

· · · · · ·

Tip

·

·

· · ·

·

· · · · ·

·

·

·

·

·

·

·

·

[71][72] [69] [37][66][67][68] [37][64][65] [62][63] [60][61] [59] [58] [57] [56] [38] [55] [54] [53] [51][52] [50] [48][49] Reference(s)

Table 1 Different schemes for one optical structural that has been investigated.

Table 2 Different schemes with two optical structures that have been investigated.

[70]

[73][74]

H. Zhou, et al.

4.2. Physical-chemical method Due to the chemical method being more difficult to control than the physical method, it is difficult to obtain the ideal lens shape through the use of the chemical corrosion method only. The ideal scheme is to combine the physical method and the chemical method. A gradient index ball lens [106] was fabricated using the modified ion-exchange and the sagging method in sodium nitrate. Without a coating, the transmittance was greater than 95% in the range of the visible and infrared light spectrum. Etching and fusion techniques have 4

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Fig. 4. A schematic and the sequence of laser direct writing fabrication [103].

been used to fabricate hyperbolic microlenses [107]. After the chemical etching method [108], arc discharge polishing [109,110] or fusion splicer heating [111] was proposed to form the lensed fibers. A conical cavity was formed on a fiber tip after chemical treatment. The intermediate was injected into the cavity and then a spherical microlens was formed [112]. A method to fabricate spherical microlenses on a tapered single mode fiber has been presented. The taper was etched by an evaporating ammonium bifluoride solution, and then a hemispheric lens was melted by a CO2 laser on the taper tip [113]. A hemispherical microlens was fabricated using a process step, which consists of exposure to a femtosecond laser, chemical etching and oxyhydrogen flame polishing [114]. The fabrication procedures have been shown in Fig. 6. Different optical structures are machined in different ways, but a fabrication process with high-performance, high-consistency, a highfabrication rate, and low-cost is a common goal being pursued by researchers.

basic idea of the beam propagation method is to calculate the field in each propagation section, step by step, on the premise of the given initial field. It is typically only used to solve for intensity and modes within shaped waveguide structures, as opposed to scattering problems. The Beam Propagation Method relies on the slowly varying envelope approximation, and is inaccurate for the modelling of discretely or quickly varying structures. The beam propagation method solves the scalar or semi-vector Helmholtz equation by applying the finite difference technique to calculate the three-dimensional spatial light field in a series of evenly spaced planes: 2

x2

+ k 02 (n2

v 2 ) A (x , y ) = ± 2jk 0 v

A (x , y ) y

(3)

where k0,n and v denote the wavenumber, refractive index and beam prorogation speed in free space, respectively, and A(x,y) is the slowly varying envelope function. The beam propagation method is typically only used to solve for intensity and the modes within shaped waveguide structures. It is employed for numerical simulation of microlens fiber coupling systems, such as a wedge shaped graded index fiber [117], a wedge shaped lensed fiber [45], a tapered graded index fiber [118]. So is the ball lens coupling system [24]. When compared with wave propagation method [119], the beam propagation method is fast but is only applicable for small apertures, while no paraxial approximation is needed for the wave propagation method but more computational effort is required.

5. Mathematical modeling method To make the coupling efficiency calculation reliable, the most important thing is to describe the law of light transmission through an interface of different shapes. 5.1. Beam propagation method The beam propagation method [115,116] is one of the most popular methods in the research and design of optical waveguide devices. The

Fig. 5. The simplified fabrication process for producing the proposed microlensed fibers [105]. 5

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Fig. 6. Schematic of the fabrication procedures of a hemispherical microlens [114].

Fig. 8. Diagram of the multiple optical elements system.

coupling surface can be obtained from the light transformation matrix. Differing from the methods above, it simplifies the coupling efficiency calculation and does not need to use cumbersome numerical integrations. Initially, the researchers derived the ABCD ray matrices for freespace transmission, thin lens transmission, interfacial refraction and spherical refraction [124]. Based on phase matching at the boundary, the ABCD matrices have been deduced for some special optical surfaces. Two assumptions were made in this analysis: one is that the beam diameter is always smaller than the radii of curvature of the wave front and the optical surface. Therefore, it just has to consider the wave front parameters along the transverse coordinates to the second order. The other is that the principal axis of the shape surface is located in the incident plane. Used this way, the ABCD matrices of the reflection and refraction of Gaussian light beams have been deduced at ellipsoidal surfaces [125], hyperbolic surfaces [66], parabola surfaces [60] and elliptical parabolic surfaces [40]. Alternatively, based on Snell’s law of refraction under paraxial approximation, using the geometric analysis method, the ABCD matrices at these surfaces [59,61,67,68,126,127] can be deduced as well. Even in an elliptical core fiber [71–74], a graded index fiber [56–58], a dispersion shifted fiber [38,64], and a graded index fiber with an upside down micro-lens [48–51,55], this method still shows excellent adaptability.

Fig. 7. Diagram of a ray of light being traced through a medium with a varying refractive index.

5.2. Ray tracing method Ray tracing [120,121] is a method for calculating the path of waves or particles through a system with regions of varying propagation velocity, absorption characteristics, and reflecting surfaces. Fig. 7 has shown the diagram of a ray of light being traced through a medium with a varying refractive index. Geometric ray tracing is used to describe the propagation of light rays through a lens system or an optical instrument, allowing the image-forming properties of the system to be modeled. It accurately applies the refraction law to each refraction surface. It needs to calculate the region in advance, where the light can meet the total reflection condition on the end surface of the fiber, and then integrate the light power of the region. The optical path length from the light source is used to compute the phase. The ray tracing method was used to obtain the optimal profile of the microlens. The Fresnel diffraction theory was used to represent the laser mode fields. A phase delay, caused by the special shape of the fiber tip, was added to the laser field. With the help of the fiber’s fundamental mode field, the coupling efficiency can be calculated from the overlap integral. A theoretical model has been described for laser diodes coupling to a wedged lens fiber [86], a hemispherical lens fiber [37,82,122], a hyperbolic lens fiber [37,82,122], a double wedged lens fiber [41], a tapered hyperbolic lens fiber [107], and a conical wedge lens fiber [47]. In the past, ray tracing calculations were performed by hand. A simple version of ray tracing, known as ray transfer matrix analysis, has been used by researchers since the advent of the computer.

6. Summary Optical communication devices are an important foundation of the information industry. A single mode fiber coupling to a laser diode is a crucial technology for optical communication, the application of which has drawn much attention. Coupling efficiency between single mode fibers and laser diodes is a key parameter characterizing the performance and reliability of optical fiber communication systems. Increasing the coupling efficiency is a crucial research subject. In this paper, the technology of a single mode fiber coupling to a semiconductor laser diode has been summarized and the latest developments in the bulk optics coupling scheme and the microlens fiber coupling scheme have been reviewed. The review focused on optimizing the optical structure and the coupling parameters to improve the coupling efficiency and the packaging performance. The advanced manufacturing technology, as well as the common modeling methods and applications of coupling systems, have also been reviewed. The bulk optics coupling scheme and the microlens fiber coupling scheme have also been covered. Establishing a suitable coupling model is helpful in improving the computational efficiency and accuracy. The ABCD matrix method

5.3. Ray transfer matrix method The ray transfer matrix, known as the ABCD matrix [123], is a type of ray tracing technique used in the design of some optical systems, particularly lasers. It involves the construction of a ray transfer matrix which describes the optical system; the tracing of a light path through the system can then be performed by multiplying this matrix by a vector representing the light ray. A diagram of the multiple optical elements system has been shown in Fig. 8. The total of the transformation matrix can be expressed as T = Tn Tn 1···T1. The propagation of a Gaussian beam follows the ABCD matrix of paraxial rays. By means of this method, the light field expression of the 6

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effectively simplifies the calculation process of coupling efficiency. However, the acquisition of the ray transmission matrix for complex curved surfaces is a relatively large project. For this kind of problem, the beam propagation method provides a more comprehensive solution; better modeling methods need to be further explored. In order to make the optical power loss as small as possible, the optical structure between a single mode fiber and a laser diode has been optimized. However the encapsulation performance of the system and the machining performance of the optical structure are also important indicators when measuring the quality of an optical system. A superior optical coupling system should also be easy to process and package. A hyperbolic-shaped fiber is the best microlens fiber in theory; several fabrication processes for it have been proposed. However new forms of the optical structure remain to be explored.

[19] [20] [21] [22] [23] [24] [25]

Declaration of Competing Interest

[26]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[27]

Acknowledgments

[28]

Supported by the National Key R＆D Program of China (Grant No. 2017YFB1104800), and the National Natural Science Foundation of China (Grant No. 51575534).

[29] [30] [31]

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