阅读说明：本技术 一种涡旋光束与光纤高效耦合系统及方法 (Vortex light beam and optical fiber efficient coupling system and method ) 是由 俞琳 于 2021-08-10 设计创作，主要内容包括：本发明公开了一种涡旋光束与光纤耦合系统及方法,属于光通信技术领域。本系统包括测距装置,本方法基于该系统,利用测距装置测量发射端和接收端之间的距离,综合衍射展宽和大气湍流展宽,获得涡旋光束经空间传输后的扩展光斑半径,以确定可变焦透镜的焦距,使得空间涡旋光束经过透镜聚焦后,能与光纤涡旋模式的光斑尺寸匹配,提高耦合效率；同时,本方法采用拉盖尔-高斯涡旋光束作为空间传输的涡旋光束,抛物型渐变少模光纤作为接收光纤,来获得大气和光纤两种介质中涡旋模式的匹配,本方法简便易行,实施装置构造简单,对不同传输距离下涡旋光束的光纤耦合适应性强,具有良好的应用前景。(The invention discloses a vortex light beam and optical fiber coupling system and method, and belongs to the technical field of optical communication. The system comprises a distance measuring device, and the method is based on the system, measures the distance between a transmitting end and a receiving end by using the distance measuring device, integrates diffraction broadening and atmospheric turbulence broadening, obtains the expanded light spot radius of a vortex light beam after spatial transmission, and determines the focal length of a variable-focus lens, so that the spatial vortex light beam can be matched with the light spot size of an optical fiber vortex mode after being focused by the lens, and the coupling efficiency is improved; meanwhile, the Laguerre-Gaussian vortex beam is used as the vortex beam for space transmission, the parabolic gradually-changed few-mode optical fiber is used as the receiving optical fiber, and the matching of vortex modes in two media of atmosphere and the optical fiber is obtained.)

1. A vortex beam and optical fiber high-efficiency coupling system is characterized by comprising a semiconductor laser, a spiral phase plate, a variable-focus lens, a few-mode optical fiber, a distance measuring device and a computer;

the distance measuring device is used for measuring the distance from the light beam transmitting end to the receiving end; the computer is respectively connected with the variable-focus lens and the distance measuring device and is used for collecting the measuring result of the distance measuring device, calculating the optimal focal length of the variable-focus lens on the basis and controlling and adjusting the focal length of the variable-focus lens; and the emergent light beam of the semiconductor laser sequentially passes through the spiral phase plate, turbulent atmosphere and the variable focus lens and is finally received by the few-mode optical fiber, so that the coupling of the vortex light beam and the optical fiber is realized.

2. The system of claim 1, wherein the few-mode fiber is a parabolic tapered few-mode fiber.

3. The system of claim 2, wherein the ranging device comprises: a laser range finder and a range target; after the light beam emitted by the laser range finder is reflected by the range finding target, the light beam returns to be received by the laser range finder; the computer is connected with the laser range finder.

4. The system of claim 3, wherein the reflective surface of the range target and the transmissive surface of the spiral phase plate are in the same vertical plane, and the exit surface of the laser range finder and the entrance surface of the variable focus lens are in the same vertical plane.

5. A method for efficiently coupling vortex light beams with optical fibers, which is implemented based on the vortex light beam and optical fiber efficient coupling system of claim 4, and comprises the following steps:

the method comprises the following steps: measuring the distance between a transmitting end and a receiving end by using a distance measuring device;

step two: calculating the expanded spot radius of the Laguerre-Gaussian vortex light beam at the transmission distance according to the distance between the transmitting end and the receiving end;

step three: calculating the optimal focal length value of the variable focus lens according to the extended spot radius of the Laguerre-Gaussian vortex beam obtained in the step two;

step four: according to the optimal focal length value obtained by calculation, the focal length of the variable-focus lens and the distance from the parabolic gradually-changed few-mode optical fiber to the lens are adjusted, so that the parabolic gradually-changed few-mode optical fiber is positioned at the focal point of the variable-focus lens;

step five: the output light beam of the semiconductor laser is converted into a Laguerre-Gaussian vortex light beam through a spiral phase plate, and after spatial transmission is carried out through turbulent atmosphere, the output light beam is received by a variable focus lens and focused into a parabolic gradually-changed few-mode optical fiber, so that the vortex light beam is coupled with the optical fiber.

6. The method of claim 5, wherein the optimal focal length f of the variable focus lens_optComprises the following steps:

wherein, w_zExtended spot radius, w, for Laguerre-Gaussian vortex beam_f0The optical fiber is a basic mode beam waist of a parabolic gradient few-mode optical fiber, lambda is a beam wavelength, p is a radial order, and l is an angular order.

7. The method of claim 6, wherein the extended spot radius w of the Laguerre-Gaussian vortex beam_zComprises the following steps:

wherein z is the distance from the transmitting end to the receiving end, w₀Is the fundamental mode beam waist of a Laguerre-Gaussian vortex beam, kappa is the three-dimensional space wave number, and phi (kappa) is the turbulent refractive index fluctuation power spectrum.

8. The method of claim 7, wherein the parabolic tapered few-mode fiber has a fundamental mode beam waist w_f0Comprises the following steps:

wherein r is₀Is a parabolic gradually-changed few-mode optical fiber core radius, n₀The refractive index of the central axis of the optical fiber, and beta is the refractive index dispersion parameter of the optical fiber.

9. The method of claim 8, wherein the Laguerre-Gaussian vortex beam w₀＝0.02m，λ＝1.55μm，p＝0，l＝1。

10. The method of claim 9, wherein the parabolic tapered few-mode fiber r₀＝30μm，n₀＝1.45，β＝0.01。

Technical Field

The invention relates to a vortex light beam and optical fiber efficient coupling system and method, and belongs to the technical field of optical communication.

Background

In air-ground laser communication, a space beam is transmitted through the atmosphere, and then is usually coupled into an optical fiber, amplified by power, and then subjected to signal detection or land transmission. Therefore, improving the fiber coupling efficiency of the spatial light beam is very important in the construction of the air-ground communication system. The current research mostly focuses on the coupling of spatial gaussian beam and single-mode fiber, and has formed more mature techniques and devices. However, in the face of increasing large data transmission requirements, the transmission capacity of single-mode optical fiber is approaching to its nonlinear shannon limit, and the space for continuous promotion is not large. Compared with a single mode fiber which can only transmit a single laser mode (namely, a fundamental mode Gaussian beam), the few-mode fiber can allow a plurality of mutually orthogonal high-order laser modes carrying different orbital angular momentum to be transmitted simultaneously, so that the communication capacity can be greatly improved, and the optical fiber has attractive application prospects. The vortex beam is a special beam with spiral phase wavefront and carrying orbital angular momentum, so that the vortex beam is used for replacing a Gaussian beam to study the coupling of the Gaussian beam and a few-mode optical fiber, and the study is concerned by academia and industry.

In the coupling of vortex beams and few-mode fibers, the preference for beam and fiber types is crucial. The vortex light beams are various in types, the corresponding light field modes are different, and the vortex light field modes supporting transmission in the few-mode optical fibers with different structures are different. In the current report, the Laguerre-Gaussian vortex beam and the conventional step type few-mode optical fiber quickly receive attention by virtue of the advantages of easy construction and wide application, and obtain a better coupling effect when the order of the vortex optical field mode is lower. But the defects are that the vortex light field modes of the two can not be completely matched, and the matching degree is reduced along with the increase of the mode order, so that the coupling efficiency of the high-order mode is obviously reduced.

In addition, when the spatial light beam is transmitted in the atmosphere, the spatial light beam is influenced by diffraction and atmospheric turbulence, and phenomena such as spot expansion, beam drift, mode crosstalk and the like are caused. The light spot expansion degree of the vortex light beam is different with the transmission environment, but the light spot size of the vortex mode of the optical fiber is certain due to the limitation of the aperture of the optical fiber. Therefore, for a vortex beam with a large spot spread, it is likely that it cannot be completely focused into the fiber when a lens with a fixed focal length is used, resulting in a decrease in coupling efficiency. The variable focus lens is adopted to replace a fixed focal length lens, which is beneficial to matching of vortex light beams and optical fiber vortex modes on the spot size, and the key is how to quickly and accurately determine the optimal focal length of the variable focus lens, while no specific scheme for determining the optimal focal length when vortex light beams are coupled with optical fibers is provided at present.

Disclosure of Invention

The invention provides a system and a method for efficiently coupling vortex beams and optical fibers, aiming at solving the problem that the existing vortex beams and optical fibers cannot be efficiently coupled.

It is a first object of the present invention to provide a vortex beam and optical fiber efficient coupling system, the system comprising: the device comprises a semiconductor laser, a spiral phase plate, a variable focus lens, a few-mode optical fiber, a distance measuring device and a computer;

the distance measuring device is used for measuring the distance from the light beam transmitting end to the receiving end; the computer is respectively connected with the variable-focus lens and the distance measuring device and is used for collecting the measuring result of the distance measuring device, calculating the optimal focal length of the variable-focus lens on the basis and controlling and adjusting the focal length of the variable-focus lens; and the emergent light beam of the semiconductor laser sequentially passes through the spiral phase plate, turbulent atmosphere and the variable focus lens and is finally received by the few-mode optical fiber, so that the coupling of the vortex light beam and the optical fiber is realized.

Optionally, the few-mode fiber is a parabolic tapered few-mode fiber.

Optionally, the distance measuring device includes: a laser range finder and a range target; after the light beam emitted by the laser range finder is reflected by the range finding target, the light beam returns to be received by the laser range finder; the computer is connected with the laser range finder.

Optionally, the reflection surface of the ranging target and the transmission surface of the spiral phase plate are located in the same vertical plane, and the exit surface of the laser range finder and the entrance surface of the variable focus lens are located in the same vertical plane.

The second objective of the present invention is to provide a method for efficiently coupling a vortex beam with an optical fiber, which is implemented based on the above system for efficiently coupling a vortex beam with an optical fiber, and the method includes:

the method comprises the following steps: measuring the distance between a transmitting end and a receiving end by using a distance measuring device;

step two: calculating the expanded spot radius of the Laguerre-Gaussian vortex light beam at the transmission distance by using the distance between the transmitting end and the receiving end;

step three: calculating the optimal focal length value of the variable focus lens according to the extended spot radius of the Laguerre-Gaussian vortex beam obtained in the step two;

step four: according to the optimal focal length value obtained by calculation, the focal length of the variable-focus lens and the distance from the parabolic gradually-changed few-mode optical fiber to the lens are adjusted, so that the parabolic gradually-changed few-mode optical fiber is positioned at the focal point of the variable-focus lens;

step five: the output light beam of the semiconductor laser is converted into a Laguerre-Gaussian vortex light beam through a spiral phase plate, and after spatial transmission is carried out through turbulent atmosphere, the output light beam is received by a variable focus lens and focused into a parabolic gradually-changed few-mode optical fiber, so that the vortex light beam is coupled with the optical fiber.

Optionally, the optimum focal length f of the variable focus lens_optComprises the following steps:

wherein, w_zExtended spot radius, w, for Laguerre-Gaussian vortex beam_f0The optical fiber is a basic mode beam waist of a parabolic gradient few-mode optical fiber, lambda is a beam wavelength, p is a radial order, and l is an angular order.

Optionally, the extended spot radius w of the laguerre-gaussian vortex beam_zComprises the following steps:

wherein z is the distance from the transmitting end to the receiving end, w₀Is the Laguerre-Gaussian vortex beam fundamental mode beam waist, kappa is the three-dimensional space wave number, and phi (kappa) is the turbulent refractive index fluctuation power spectrum.

Optionally, the base mode beam waist w of the parabolic tapered few-mode fiber_f0Comprises the following steps:

wherein r is₀Gradually changing few-mode light in a parabolic shapeCore radius of fiber, n₀The refractive index of the central axis of the optical fiber, and beta is the refractive index dispersion parameter of the optical fiber.

Optionally, the laguerre-gaussian vortex beam w₀＝0.02m，λ＝1.55μm，p＝0，l＝1。

Optionally, the parabolic tapered few-mode optical fiber r₀＝30μm，n₀＝1.45，β＝0.01。

The invention has the beneficial effects that:

the system comprises a semiconductor laser, a spiral phase plate, a variable focus lens, a parabolic gradually-changed few-mode optical fiber, a distance measuring device and a computer; the system measures the distance from the transmitting end to the receiving end of the light beam by using the distance measuring device; based on the system, the invention also provides a method for efficiently coupling the vortex light beam and the optical fiber, when the coupling of the vortex light beam and the optical fiber is realized, the distance from the transmitting end to the receiving end is measured by using the distance measuring device, according to the distance measuring result, the computer can quickly and accurately calculate the optimal focal length of the variable-focus lens under different transmission distances and adjust the focal length, thereby improving the matching degree of the light spot size of the vortex light beam and the vortex mode of the optical fiber, reducing the adverse effect of the light spot expansion of the vortex light beam during the space transmission and greatly improving the coupling efficiency; meanwhile, the emergent light beam of the semiconductor laser generates a Laguerre-Gaussian vortex light beam through the spiral phase plate, the receiving optical fiber selects the parabolic gradually-changed few-mode optical fiber, the vortex mode can be completely matched between the receiving optical fiber and the parabolic gradually-changed few-mode optical fiber, and the coupling efficiency is further improved.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a schematic diagram of a system for achieving efficient coupling of a vortex beam to an optical fiber.

Fig. 2 is a light field distribution diagram of a laguerre-gaussian vortex beam.

FIG. 3 is a refractive index profile of a parabolic tapered few-mode fiber.

FIG. 4 is a graph of fiber coupling efficiency as a function of focal length of a variable focus lens.

Figure 5 is a graph of the variation of the optimum focal length of a variable focus lens for different transmission distances.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The first embodiment is as follows:

the embodiment provides a system for efficiently coupling a vortex light beam and an optical fiber, and as shown in fig. 1, the system comprises a semiconductor laser 1, a spiral phase plate 2, a variable focus lens 3, a parabolic tapered few-mode optical fiber 4, a distance measurement target 5, a laser distance meter 6 and a computer 7, wherein the devices 1, 2 and 5 are positioned at a transmitting end, the devices 3, 4, 6 and 7 are positioned at a receiving end, and the device 7 is connected with control terminals of the devices 3 and 6 through data lines; the reflecting surface of the distance measuring target 5 and the transmitting surface of the spiral phase plate 2 are positioned in the same vertical plane, and the emergent surface of the laser distance measuring instrument 6 and the incident surface of the variable-focus lens 3 are positioned in the same vertical plane.

The distance between the transmitting end and the receiving end is measured by the laser range finder 6: after the Gaussian beam emitted by the range finder 6 is reflected by the range finding target 5, the Gaussian beam returns to be received by the range finder 6, and the range finding is realized by recording the time required by the Gaussian beam to go back and forth once and multiplying the time by the speed of light.

After the computer 7 collects the ranging result output by the range finder 6, the extended spot radius of the transmission distance down Laguerre-Gaussian vortex beam is calculated, and on the basis, the optimal focal length of the variable focus lens 3 is calculated;

the computer 7 sends the optimal focal length value to the control terminal of the variable focal length lens 3, and adjusts the focal length of the variable focal length lens 3 in real time. Correspondingly, the position of the parabolic gradually-changed few-mode optical fiber 4 is adjusted accordingly, so that the parabolic gradually-changed few-mode optical fiber 4 is always positioned at the focus of the variable-focus lens 3 to receive the focused light beam of the variable-focus lens 3.

A Gaussian beam emitted by the semiconductor laser 1 is incident on the spiral phase plate 2 to load a vortex phase, and emergent light is converted into a Laguerre-Gaussian vortex beam. After being transmitted by an atmospheric medium containing turbulence, the vortex light beam is received by the variable-focus lens 3 and focused into the parabolic gradually-changed few-mode optical fiber 4, so that the vortex light beam is coupled with a vortex mode of the optical fiber.

Example two:

the embodiment provides a method for efficiently coupling vortex beams with optical fibers.

The vortex light beams are various in types, the corresponding vortex mode light field distributions are different, and the vortex modes which can be transmitted in the few-mode optical fibers with different fiber core refractive index distributions are different, so that the selection of the vortex light beams with high mode matching degree and the few-mode optical fibers is very important for improving the coupling efficiency.

The Laguerre-Gaussian vortex beam is easy to generate, excellent in property and wide in application, has a radial order p and an angular order l, is suitable for combining the multiplexing and division, and improves the communication capacity. The carried vortex mode Laguerre-Gaussian mode is shown in figure 2, the light field intensity is distributed annularly, and the number of the rings is p + 1; rotating one circle along the circumferential direction, the phase of the optical field changes by l times of 2 pi.

The parabolic gradually-changed few-mode optical fiber has a constant cladding refractive index and a parabolic core refractive indexThe refractive index is maximum at the central axis (r ═ 0), as shown in fig. 3. The intrinsic mode of the few-mode optical fiber under the refractive index distribution is solved by utilizing a Helmholtz equation, so that the parabolic gradually-changed few-mode optical fiber supports transmission of a Laguerre-Gaussian mode and is used for coupling the Laguerre-Gaussian vortex light beam to realize complete matching of a vortex mode.

The spot radius expands as the vortex beam is subjected to diffraction and atmospheric turbulence as it travels through the air space. Beam waist of w for the base mode₀Laguerre-Gauss vortex with wavelength of lambda, radial order of p and angular order of lA rotating beam of light transmitted to a spot radius w at z-distance_zCan be expressed as

The first term in the root of the above equation characterizes the spot expansion caused by diffraction, and the second term characterizes the spot expansion caused by atmospheric turbulence, where κ is the three-dimensional space wavenumber, Φ (κ) is the turbulent refractive index fluctuation power spectrum, and Φ (κ) is a characteristic parameter describing turbulence. At w_zIn (1), diffraction broadening dominates. On the other hand, the spot size of the vortex mode in the fiber is fixed, limited by the fiber aperture. For a core radius of r₀Refractive index n at central axis₀Parabolic gradient optical fiber with refractive index dispersion parameter beta, and light spot radius w of vortex mode of optical fiber_fCan be expressed as

Wherein w_f0Is the fiber fundamental mode beam waist.

At this point, if a fixed focal length lens is used to couple the Laguerre-Gaussian vortex beam into the fiber, w_zAnd w_fThe mismatch between them can lead to a reduction in coupling efficiency.

FIG. 4 refractive index fluctuation power spectrum according to atmospheric turbulenceWherein κ₀＝2π/L₀，κ_m＝5.92/l₀) Simulate w₀Laguerre gaussian vortex beam with λ 1.55 μm, p 0, L1 at turbulent outer scale factor L₀1m, inner scale factor l₀1mm, refractive index structure constantIs coupled to r by a variable focus lens after transmitting z 1000m in the atmosphere₀＝30μm，n₀In the parabolic tapered fiber with the wavelength beta of 1.45 and the wavelength beta of 0.01, the existence of mode crosstalk is considered, the coupling efficiency of a signal mode changes along with the focal length of a lens, and the optimal selection of the focal length is favorable for improving the coupling efficiency.

In order to select the optimal focal length and realize the size matching of the space beam and the optical fiber light spot, the embodiment firstly adopts a laser range finder to measure the distance of the Laguerre-Gaussian vortex beam transmitted from the spiral phase plate at the transmitting end to the variable-focus lens at the receiving end, the influence of diffraction broadening and turbulence broadening is comprehensively considered, and the extended light spot radius w of the Laguerre-Gaussian vortex beam at the incident plane of the lens can be obtained by the formula (1)_z。

Then, by using a light transmission matrix method in geometric optics, the fact that when the vortex mode of the optical fiber is transmitted to the incident plane of the lens from the back of the end face of the optical fiber at the focal point of the lens, the radius of a light spot is w_fBecome intoAnd isThe expression of (a) is:

if it isThe expanded light spot of the Laguerre-Gaussian vortex light beam can be matched with the light spot size of the vortex mode of the optical fiber after being focused by the lens, and the optimal focal length f of the variable-focus lens is obtained_optIs expressed as

By combining the formula (1) and the formula (2), f can be calculated_optThe value of (c).

FIG. 5 shows the Laguerre-Gaussian vortex beam coupling at different transmission distances zOptimum focal length of lens required in parabolic tapered few-mode fiber, wherein beam parameter (w)₀λ, p, l), fiber parameters (r)₀,n₀Beta) and atmospheric turbulence parametersIs consistent with FIG. 4, it can be seen that as z increases, f_optThe value of (c) also needs to be increased accordingly.

Calculating f from the measured z_optAnd then, the focal length of the lens is adjusted in real time through a control terminal of the zoom lens, and meanwhile, the distance from the optical fiber to the lens is correspondingly changed, so that the optical fiber is always positioned at the focal point of the lens.

At the moment, the Gaussian beam emitted by the semiconductor laser is converted into a Laguerre-Gaussian vortex beam through the spiral phase plate, when the vortex beam is transmitted to a receiving end in a turbulent atmospheric space, the vortex beam is focused into the parabolic tapered few-mode optical fiber through the variable-focus lens adjusted to be in the optimal state, and the vortex mode and the light spot size are matched, so that the vortex beam and the optical fiber can be efficiently coupled. As can also be seen from fig. 4, when the coupling system is adjusted to the optimal state, the coupling efficiency can be significantly improved, and the practical value is very high.

Some steps in the embodiments of the present invention may be implemented by software, and the corresponding software program may be stored in a readable storage medium, such as an optical disc or a hard disk.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.