阅读说明：本技术 一种多波长拉曼激光器 (Multi-wavelength Raman laser ) 是由 葛文琦 常慧 肖红 柯常军 张鸿博 樊仲维 于 2021-07-26 设计创作，主要内容包括：本公开的多波长拉曼激光器,包括基频激光光源1、半波片2、透镜组3、反射镜4和条形受激拉曼散射晶体5；基频激光光源1用于发射单一波长的基频激光；半波片2为激光偏振调节器件,用于将入射的基频激光的偏振方向调节至水平偏振方向；透镜组3用于缩放经半波片2调节为水平偏振的基频激光的光束口径与条形受激拉曼散射晶体5的口径相匹配,并透射到所述反射镜4；反射镜4用于将入射的基频激光进行反射,使基频激光垂直入射到条形受激拉曼散射晶体5；条形受激拉曼散射晶体5用于使垂直入射的基频激光发生一阶受激拉曼散射效应并进行折线型传输,得到多波长受激拉曼散射激光。本公开的多波长拉曼激光器具有一阶受激拉曼散射效率高,产生多波长激光偏振相同,稳定性好,实用性强等优点。(The multi-wavelength Raman laser comprises a fundamental frequency laser light source 1, a half-wave plate 2, a lens group 3, a reflector 4 and a strip-shaped stimulated Raman scattering crystal 5; the fundamental frequency laser light source 1 is used for emitting fundamental frequency laser with single wavelength; the half-wave plate 2 is a laser polarization adjusting device and is used for adjusting the polarization direction of the incident fundamental frequency laser to the horizontal polarization direction; the lens group 3 is used for zooming the beam caliber of the fundamental frequency laser which is adjusted to be horizontally polarized by the half-wave plate 2 to be matched with the caliber of the strip stimulated Raman scattering crystal 5 and transmitting the beam to the reflector 4; the reflector 4 is used for reflecting the incident fundamental laser to enable the fundamental laser to vertically enter the strip-shaped stimulated Raman scattering crystal 5; the strip-shaped stimulated Raman scattering crystal 5 is used for enabling the vertically incident fundamental frequency laser to generate a first-order stimulated Raman scattering effect and perform fold line type transmission to obtain multi-wavelength stimulated Raman scattering laser. The multi-wavelength Raman laser has the advantages of high first-order stimulated Raman scattering efficiency, same polarization of generated multi-wavelength laser, good stability, strong practicability and the like.)

1. A multi-wavelength raman laser, characterized in that the laser comprises: the device comprises a fundamental frequency laser light source 1, a half-wave plate 2, a lens group 3, a reflector 4 and a strip-shaped stimulated Raman scattering crystal 5;

the fundamental frequency laser light source 1 is used for emitting fundamental frequency laser with a single wavelength;

the half-wave plate 2 is a laser polarization adjusting device and is used for adjusting the polarization direction of incident fundamental frequency laser to a horizontal polarization direction;

the lens group 3 is used for zooming the beam aperture of the fundamental laser which is adjusted to be horizontally polarized by the half-wave plate 2 to match with the aperture of the strip stimulated Raman scattering crystal 5 and transmitting the beam to the reflector 4;

the reflector 4 is used for reflecting the incident fundamental frequency laser to enable the fundamental frequency laser to vertically enter the strip-shaped stimulated Raman scattering crystal 5;

the strip-shaped stimulated Raman scattering crystal 5 is used for enabling the vertically incident fundamental frequency laser to generate a first-order stimulated Raman scattering effect and perform broken line type transmission to obtain multi-wavelength stimulated Raman scattering laser.

2. The multiwavelength Raman laser of claim 1, wherein the lens assembly 3 is coated with a fundamental laser antireflection film comprising one or more meniscus lenses.

3. The multiwavelength Raman laser of claim 1, wherein the mirror 4 is coated with a fundamental laser highly reflective film.

4. The multiwavelength raman laser according to claim 1, wherein the stimulated raman scattering crystal 5 in the form of a stripe is an anisotropic biaxial crystal and the material is a nonlinear potassium gadolinium tungstate crystal.

5. The multiwavelength Raman laser according to claim 4, wherein the strip-shaped stimulated Raman scattering crystal 5 is a hexahedron in which two side surfaces are parallelograms or isosceles trapezoids and the remaining four surfaces are optical surfaces which are perpendicular to the two side surfaces.

6. The multiwavelength Raman laser of claim 5, wherein when both sides are parallelograms, the acute angle of the parallelogram is 45 °; when the two side surfaces are isosceles trapezoids, the base angle of the isosceles trapezoids is 45 degrees.

7. The multiwavelength Raman laser of claim 5, wherein two opposing optical surfaces of the four optical surfaces are fundamental laser transmission surfaces coated with fundamental laser and first-order stimulated Raman scattering laser antireflection coatings; the other two opposite optical surfaces are base frequency laser reflecting surfaces, and are plated with base frequency laser and a first-order stimulated Raman scattering laser high reflecting film.

8. The multiwavelength raman laser of claim 7, wherein three mutually perpendicular principal axes of optical refractive index of the strip-shaped stimulated raman scattering crystal 5 are a first principal axis, a second principal axis, and a third principal axis, respectively; wherein the first principal axis is perpendicular to both side surfaces, parallel to the four optical surfaces; the second main shaft is parallel to the transmission surface of the fundamental frequency laser and forms an angle of 45 degrees with the reflection surface of the fundamental frequency laser; the third main shaft is perpendicular to the fundamental frequency laser transmission surface and forms a 45-degree angle with the fundamental frequency laser reflection surface.

9. The multiwavelength raman laser according to claim 8, wherein the fundamental laser light is perpendicularly incident on a fundamental laser transmission surface of the strip-shaped stimulated raman scattering crystal 5, the fundamental laser light propagates along the second principal axis or the third principal axis, the polarization direction is the third principal axis or the second principal axis, multiple reflections are performed between the two fundamental laser reflection surfaces, zigzag transmission is realized, and the transmission output is performed through the other fundamental laser transmission surface of the strip-shaped stimulated raman scattering crystal 5.

10. The multiwavelength raman laser of claim 1, wherein a pulse width of the fundamental laser is in the order of femtoseconds to microseconds; the wavelength range is from ultraviolet to mid-infrared.

Technical Field

The invention belongs to the technical field of laser, and particularly relates to a multi-wavelength Raman laser which realizes output of multi-wavelength first-order stimulated Raman scattering laser by means of zigzag transmission of laser in a strip-shaped stimulated Raman scattering crystal.

Background

The laser technology has been widely applied in many fields such as basic scientific research, clinical medicine, information communication, industrial technology and the like, and is a powerful tool for exploring and developing the advanced fields. With the development of the fields of laser differential detection, laser communication, spectral analysis, laser ranging and the like, the laser with a single wavelength cannot meet the application requirements, and thus a laser capable of generating multiple wavelengths is required. The laser is limited by the energy level structure of the laser gain material, and can only output laser of certain specific wave bands, so that the multi-wavelength laser generation technology with simple structure and reliable performance has important scientific significance.

At present, multiple wavelength lasers are obtained by a plurality of means, mainly including: nonlinear sum frequency, nonlinear frequency multiplication, optical parametric oscillation, stimulated Raman scattering, laser direct modulation and the like. Among them, Stimulated Raman Scattering (SRS) is a third-order nonlinear effect, and is an effective means for realizing laser frequency conversion. When the photons of the fundamental frequency excitation laser interact with the nonlinear Raman crystal, the excited Raman scattering laser photons are generated and simultaneously emit or absorb a phonon. The frequency of the stimulated raman scattering laser differs from the frequency of the fundamental excitation laser by a frequency difference, which is called nonlinear frequency shift. The frequency of the stimulated Raman scattering laser is determined by the incident fundamental frequency laser frequency and the Raman medium, and the frequency shift amount of each nonlinear frequency conversion depends on the Raman characteristic spectral line of the nonlinear Raman crystal. Through one or more times of stimulated Raman scattering frequency conversion, a series of new laser wavelengths can be obtained. Compared with the technical means such as nonlinear frequency doubling, optical parametric oscillation and the like, the stimulated Raman scattering phase matching condition is not strict, and laser wavelength which cannot be obtained due to the fact that the parametric oscillation is limited by phase matching can be obtained.

At present, the commonly used raman laser implements multi-wavelength output, mainly including: scheme for obtaining multi-wavelength output by using multi-order stimulated Raman scattering process or cascade stimulated Raman scattering process [ R.P. Mildren, et al. Effect, all-solid-state, Raman laser in the yellow, orange and red, optics express,2004,12, 785-. However, in the scheme, the nonlinear conversion efficiency of the high-order stimulated raman scattering is low, the nonlinear conversion processes of the various orders are mutually influenced, and the stability of the output stimulated raman scattering laser is poor, which is not beneficial to the determination of practical application.

A scheme of multi-wavelength output is obtained by utilizing first-order frequency conversion of a plurality of Raman characteristic spectral lines [ Duan, Y., equivalent. RbTiOPO4 captured Raman operation with multiple Raman frequency shifted by Q-switched Nd: YAlO3 laser Sci Rep,2016,6,33852 ]. The polarization direction of the incident laser of the scheme is along one main axis direction of the Raman crystal, and a plurality of Raman frequency shift characteristic spectral lines exist in the main axis direction. The disadvantages of this scheme are: the stimulated Raman scattering lasers of various wavelengths corresponding to different Raman frequency shift characteristic spectral lines compete with each other, and the stimulated Raman scattering lasers of each wavelength are poor in power stability and practicability.

A scheme for obtaining multi-wavelength output by utilizing various laser nonlinear crystals [ Hongbin Shen, et al, Simultaneous minor-wavelength operation of Nd: YVO4 self-Raman laser at 1524nm and undoped GdVO4 Raman laser at 1522nm, optics. letters,2012,37, 4113-. The disadvantages of the scheme are that: increasing the number of nonlinear stimulated raman scattering crystals increases the complexity and cost of the system.

The scheme of obtaining multi-wavelength output by utilizing the different stimulated Raman scattering characteristic spectral lines in different polarization directions of the anisotropic crystal [ Shuanghong Ding, et al. the theoretical and experimental research on the multi-frequency Raman converter with KGd (WO4)2crystal, optics. express,2005,13, 10120-. The electric field polarization direction of incident fundamental laser is arranged between two mutually perpendicular main axis directions of the nonlinear Raman crystal, the incident laser forms two independent polarization components in the two main axis directions of the Raman crystal, stimulated Raman scattering is respectively generated in the two main axis directions of polarization, and the wavelength of the scattered laser is determined by Raman frequency shift characteristic spectral lines in the two main axis directions of polarization of the crystal. Because the Raman frequency shift characteristic spectral lines in different polarization directions are different, stimulated Raman scattering laser with two different wavelengths is generated, and therefore multi-wavelength operation is formed. The disadvantages of this scheme are: the incident laser forms two independent polarization components in the two main axis directions of the nonlinear Raman crystal to respectively generate stimulated Raman scattering, so that the laser intensity in a single polarization direction is weakened, and the nonlinear frequency conversion efficiency is reduced; meanwhile, the polarization directions of the lasers with the new wavelengths are different, so that the lasers are inconvenient to use in applications related to laser polarization.

Disclosure of Invention

The invention overcomes one of the defects of the prior art, and provides the multi-wavelength Raman laser which has the advantages of high first-order stimulated Raman scattering efficiency, good stability, strong practicability and the like.

According to an aspect of the present disclosure, there is provided a multi-wavelength raman laser, the laser including: the device comprises a fundamental frequency laser light source 1, a half-wave plate 2, a lens group 3, a reflector 4 and a strip-shaped stimulated Raman scattering crystal 5;

the fundamental frequency laser light source 1 is used for emitting fundamental frequency laser with a single wavelength;

the half-wave plate 2 is a laser polarization adjusting device and is used for adjusting the polarization direction of incident fundamental frequency laser to a horizontal polarization direction;

the lens group 3 is used for zooming the beam aperture of the fundamental laser which is adjusted to be horizontally polarized by the half-wave plate 2 to match with the aperture of the strip stimulated Raman scattering crystal 5 and transmitting the beam to the reflector 4;

the reflector 4 is used for reflecting the incident fundamental frequency laser to enable the fundamental frequency laser to vertically enter the strip-shaped stimulated Raman scattering crystal 5;

the strip-shaped stimulated Raman scattering crystal 5 is used for enabling the vertically incident fundamental frequency laser to generate a first-order stimulated Raman scattering effect and perform broken line type transmission to obtain multi-wavelength stimulated Raman scattering laser.

In a possible implementation manner, the lens assembly 3 is plated with a fundamental frequency laser antireflection film, and comprises one or more concave-convex lenses.

In one possible implementation, the reflector 4 is plated with a fundamental laser high reflection film.

In a possible implementation manner, the strip-shaped stimulated raman scattering crystal 5 is an anisotropic biaxial crystal and is made of a nonlinear potassium gadolinium tungstate crystal.

In one possible implementation manner, the strip-shaped stimulated raman scattering crystal 5 is a hexahedron, two side surfaces of which are parallelograms or isosceles trapezoids, and the remaining four surfaces are optical surfaces which are perpendicular to the two side surfaces.

In one possible implementation, when the two sides are parallelograms, the acute angle of the parallelogram is 45 °; when the two side surfaces are isosceles trapezoids, the base angle of the isosceles trapezoids is 45 degrees.

In a possible implementation manner, two opposite optical surfaces of the four optical surfaces are fundamental frequency laser transmission surfaces and are plated with fundamental frequency laser and first-order stimulated raman scattering laser antireflection films; the other two opposite optical surfaces are base frequency laser reflecting surfaces, and are plated with base frequency laser and a first-order stimulated Raman scattering laser high reflecting film.

In a possible implementation manner, three mutually perpendicular optical refractive index principal axes of the strip-shaped stimulated raman scattering crystal 5 are a first principal axis, a second principal axis and a third principal axis respectively; wherein the first principal axis is perpendicular to both side surfaces, parallel to the four optical surfaces; the second main shaft is parallel to the transmission surface of the fundamental frequency laser and forms an angle of 45 degrees with the reflection surface of the fundamental frequency laser; the third main shaft is perpendicular to the fundamental frequency laser transmission surface and forms a 45-degree angle with the fundamental frequency laser reflection surface.

In a possible implementation manner, the fundamental laser light is vertically incident to a fundamental laser transmission surface of the bar-shaped stimulated raman scattering crystal 5, the fundamental laser light is transmitted along the second main axis or the third main axis, the polarization direction is the third main axis or the second main axis, multiple reflections are performed between the two fundamental laser reflection surfaces, the zigzag transmission is realized, and the transmission output is performed through the other fundamental laser transmission surface of the bar-shaped stimulated raman scattering crystal 5.

In one possible implementation, the pulse width of the fundamental laser is in the order of femtoseconds to microseconds; the wavelength range is from ultraviolet to mid-infrared.

The multi-wavelength Raman laser comprises a fundamental frequency laser light source 1, a half-wave plate 2, a lens group 3, a reflector 4 and a strip-shaped stimulated Raman scattering crystal 5; the fundamental frequency laser light source 1 is used for emitting fundamental frequency laser with single wavelength; the half-wave plate 2 is a laser polarization adjusting device and is used for adjusting the polarization direction of the incident fundamental frequency laser to the horizontal polarization direction; the lens group 3 is used for zooming the beam caliber of the fundamental laser which is adjusted to be horizontally polarized by the half-wave plate 2 to be matched with the caliber of the strip-shaped stimulated Raman scattering crystal 5 and transmitting the beam to the reflector 4; the reflector 4 is used for reflecting the incident fundamental laser to enable the fundamental laser to vertically enter the strip-shaped stimulated Raman scattering crystal 5; the strip-shaped stimulated Raman scattering crystal 5 is used for enabling the vertically incident fundamental frequency laser to generate a first-order stimulated Raman scattering effect and perform fold line type transmission to obtain multi-wavelength stimulated Raman scattering laser. The fundamental laser can be transmitted along a single polarization direction all the time in the Raman crystal, two polarization components are prevented from being formed, the optical length of the fundamental laser passing through a stimulated Raman scattering medium is increased through fold line type transmission, the nonlinear gain and the conversion efficiency of stimulated Raman scattering are increased, the polarization direction of each new wavelength laser is the same as that of the fundamental laser, and the practicability is high.

Drawings

The accompanying drawings are included to provide a further understanding of the technology or prior art of the present application and are incorporated in and constitute a part of this specification. The drawings expressing the embodiments of the present application are used for explaining the technical solutions of the present application, and should not be construed as limiting the technical solutions of the present application.

FIG. 1 shows a schematic diagram of a multi-wavelength Raman laser according to an embodiment of the present disclosure;

fig. 2 shows a schematic structural diagram of a strip-shaped stimulated raman scattering crystal according to an embodiment of the present disclosure.

Detailed Description

The following detailed description of the embodiments of the present invention will be provided with reference to the accompanying drawings and examples, so that how to apply the technical means to solve the technical problems and achieve the corresponding technical effects can be fully understood and implemented. The embodiments and the features of the embodiments can be combined without conflict, and the technical solutions formed are all within the scope of the present invention.

Fig. 1 shows a schematic diagram of a multi-wavelength raman laser according to an embodiment of the present disclosure.

As shown in fig. 1, the multi-wavelength raman laser may include: the device comprises a fundamental frequency laser light source 1, a half-wave plate 2, a lens group 3, a reflector 4 and a strip-shaped stimulated Raman scattering crystal 5.

The fundamental frequency laser light source 1 may be configured to emit fundamental frequency laser light of a single wavelength. The fundamental frequency laser light source 1 can be used as a laser light source for exciting a stimulated raman scattering nonlinear process and emits pulse laser (fundamental frequency laser) with a single wavelength, the pulse width range of the fundamental frequency laser can be from femtosecond to microsecond, and the wavelength range of the fundamental frequency laser can be from ultraviolet to middle infrared spectrum.

The half-wave plate 2 is a laser polarization adjusting device, and can be used for adjusting the polarization direction of the incident fundamental laser light to the horizontal polarization direction.

And the lens group 3 is used for zooming the beam caliber of the fundamental laser which is adjusted to be horizontally polarized by the half-wave plate 2 to be matched with the caliber of the strip stimulated Raman scattering crystal 5 and transmitting the beam to the reflector 4. The lens group 3 is composed of one or more concave-convex lenses, is plated with a fundamental frequency laser antireflection film and has the function of zooming the aperture of a beam of fundamental frequency laser to be matched with the aperture of the strip-shaped stimulated Raman scattering crystal 5, so that the energy density of the fundamental frequency laser is ensured to be smaller than the damage threshold of the nonlinear crystal and larger than the threshold generated in the stimulated Raman scattering process.

The reflector 4 may be configured to reflect the incident fundamental laser light, so that the fundamental laser light perpendicularly enters the strip-shaped stimulated raman scattering crystal 5. The reflector 4 is plated with a fundamental laser high-reflection film, and can vertically irradiate fundamental laser to the strip-shaped stimulated Raman scattering crystal 5.

And the strip-shaped stimulated Raman scattering crystal 5 is used for enabling the vertically incident fundamental frequency laser to generate a first-order stimulated Raman scattering effect and perform fold line type transmission to obtain multi-wavelength stimulated Raman scattering laser.

Fig. 2 shows a schematic structural diagram of a strip-shaped stimulated raman scattering crystal according to an embodiment of the present disclosure.

In one example, the nonlinear material that can be used for the strip-shaped stimulated raman scattering crystal 5 is potassium gadolinium tungstate crystal (KGd (WO)₄)₂KGW). As shown in fig. 2, the strip-shaped stimulated raman scattering crystal 5 is an anisotropic biaxial crystal, and raman characteristic lines in the respective principal axis directions are different from each other.

As shown in fig. 2, the strip-shaped stimulated raman scattering crystal 5 is made into a hexahedron, two side surfaces of which may be parallelograms or isosceles trapezoids, and the other 4 surfaces S1, S2, S3, S4 are optical surfaces. The four optical surfaces S1, S2, S3, S4 are perpendicular to both sides. When the two side surfaces of the strip-shaped stimulated raman scattering crystal 5 are parallelograms, the acute angle of the parallelogram is 45 °. When two side surfaces of the strip-shaped stimulated raman scattering crystal 5 are isosceles trapezoids, the base angle of the isosceles trapezoids is 45 °.

In one example, two opposite optical surfaces of the four optical surfaces of the strip-shaped stimulated raman scattering crystal 5 are fundamental laser transmission surfaces and are plated with fundamental laser and first-order stimulated raman scattering laser antireflection films; the other two opposite optical surfaces are base frequency laser reflecting surfaces, and are plated with base frequency laser and a first-order stimulated Raman scattering laser high reflecting film.

For example, as shown in FIG. 2, if the optical surfaces S1 and S4 are fundamental laser transmission surfaces, the optical surfaces S1 and S4 are coated with fundamental laser and first order stimulated Raman scattering laser antireflection coating systems. Then the optical surfaces S2 and S3 are the reflection surfaces of the fundamental frequency laser, and the optical surfaces S2 and S3 are plated with the high reflection film system of the fundamental frequency laser and the first-order stimulated Raman scattering laser.

In one example, three mutually perpendicular optical refractive index principal axes of the strip-shaped stimulated raman scattering crystal 5 are a first principal axis, a second principal axis and a third principal axis respectively; the first main shaft is perpendicular to the two side surfaces and parallel to the four optical surfaces, the second main shaft is parallel to the fundamental frequency laser transmission surface and forms a 45-degree angle with the fundamental frequency laser reflection surface, and the third main shaft is perpendicular to the fundamental frequency laser transmission surface and forms a 45-degree angle with the fundamental frequency laser reflection surface.

The fundamental laser vertically enters a fundamental laser transmission surface of the strip-shaped stimulated Raman scattering crystal 5, the fundamental laser is reflected for multiple times between two fundamental laser reflection surfaces along the second main shaft or the third main shaft and the polarization direction of the third main shaft or the second main shaft, thereby realizing fold line type transmission, and the transmission and the output are realized through the other fundamental laser transmission surface of the strip-shaped stimulated Raman scattering crystal 5.

The following description will be given taking the first principal axis as the principal axis Np of optical refractive index, the second principal axis Ng as the principal axis Ng of optical refractive index, and the third principal axis Nm as the principal axis Nm of optical refractive index, but the first principal axis, the second principal axis, and the third principal axis are not limited thereto.

As shown in FIG. 2, three mutually perpendicular principal axes of optical refractive index of the strip-shaped stimulated Raman scattering crystal 5 are respectively Np, Nm and Ng axes, and the corresponding refractive indexes are respectively n_p、n_m、n_g(n_p<n_m<n_g). The optical principal axis Np is perpendicular to the side surface and parallel to the optical surfaces S1, S2, S3 and S4. The optical refractive index major axis Nm is parallel to the optical surfaces S1 and S4 at 45 ° from the optical surfaces S2 and S3. The principal optical index axis Ng is perpendicular to the optical surfaces S1 and S4 at 45 ° to the optical surfaces S2 and S3.

When fundamental laser light is perpendicularly incident on the optical surface S1, its propagation direction is along the principal optical refractive index axis Ng, and its polarization direction is along the principal optical refractive index axis Nm. After being reflected by the optical surface S3, the fundamental laser light propagates along the principal axis Nm of the optical refractive index, and the polarization direction is along the principal axis Ng of the optical refractive index. The next reflecting surface of the fundamental laser light is optical surface S2. The fundamental laser light undergoes multiple reflections between the optical surfaces S2 and S3, and undergoes zigzag-type transmission in the strip-shaped stimulated raman scattering crystal 5.

When the fundamental laser passes through the strip-shaped stimulated Raman scattering crystal 5, nonlinear frequency shift occurs due to the first-order stimulated Raman scattering effect, and laser with a new wavelength is generated. In the process of transmitting the fundamental laser by the fold line, the optical axes of the propagation direction and the polarization direction are alternately changed between the optical refractive index principal axis Ng and the optical refractive index principal axis Nm, the characteristic spectral lines of the stimulated Raman scattering are also alternately changed, namely the Raman frequency shift quantity is also at 901cm corresponding to the optical refractive index principal axis Nm^-1And 767cm corresponding to the principal axis Ng of the optical refractive index^-1Alternate between them, therefore, the corresponding two Raman frequency shift quantities (901 cm) can be obtained^-1And 767cm^-1) The stimulated raman scattering laser with two wavelengths can obtain stimulated raman scattering laser with a plurality of wavelengths in the same way.

According to the multi-wavelength Raman laser, the fundamental frequency laser is transmitted along a single polarization direction in the Raman crystal all the time, two polarization components are not formed, and the polarization direction of each generated new wavelength laser is the same as that of the fundamental frequency laser; the optical length of the fundamental laser passing through the stimulated Raman scattering medium is increased through fold line type transmission, the nonlinear gain and the conversion efficiency of the stimulated Raman scattering are increased, and the method has the advantage of strong practicability.

Although the embodiments of the present invention have been described above, the above descriptions are only for the convenience of understanding the present invention, and are not intended to limit the present invention. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.