WO2021174476A1 - 聚焦涡旋光发生器及其制备方法 - Google Patents

聚焦涡旋光发生器及其制备方法 Download PDF

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Publication number
WO2021174476A1
WO2021174476A1 PCT/CN2020/077903 CN2020077903W WO2021174476A1 WO 2021174476 A1 WO2021174476 A1 WO 2021174476A1 CN 2020077903 W CN2020077903 W CN 2020077903W WO 2021174476 A1 WO2021174476 A1 WO 2021174476A1
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phase plate
optical fiber
composite
helical phase
composite helical
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PCT/CN2020/077903
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English (en)
French (fr)
Inventor
王义平
余建
白志勇
廖常锐
刘朝
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深圳大学
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Priority to PCT/CN2020/077903 priority Critical patent/WO2021174476A1/zh
Publication of WO2021174476A1 publication Critical patent/WO2021174476A1/zh

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for

Definitions

  • the invention relates to the field of diffractive optics, in particular to a focused vortex light generator and a preparation method thereof.
  • Vortex beams have been widely used in the fields of communication, sensing and particle manipulation due to their unique spiral wavefront structure and central singularity.
  • the generation of vortex beams relied on free-space optical coupling, such as cylindrical-lens mode converters, spatial light modulators, micro-ring resonators, and spiral phase plates.
  • the vortex beam has high divergence characteristics during propagation, making it undesirable in some practical applications such as vortex optical coupling and vortex optical particle manipulation.
  • the conventional vortex beam generating device can generate a vortex beam, it does not have the ability to simultaneously focus the generated vortex beam. Therefore, it is necessary to use external optical elements to focus the vortex beam in the application.
  • the introduction of external focusing elements not only increases the volume of the entire optical system, adds a cumbersome beam collimation process, and reduces the stability of the system.
  • a focused vortex light generator and a manufacturing method thereof are provided.
  • a focused vortex light generator comprising: a single-mode optical fiber, a multi-mode optical fiber and a composite helical phase plate, the input end of the multi-mode optical fiber is butt-coupled with the single-mode optical fiber, and the output end of the multi-mode optical fiber is connected to The compound spiral phase plate is butt-coupled.
  • the multimode fiber is a graded index fiber, and the length of the graded index fiber is (2N+1)/4 times the pitch, where N is a natural number.
  • the transmittance function of the composite helical phase plate Meet the following relationships:
  • ⁇ is the wavelength of the modulated laser
  • f is the design focal length of the composite helical phase plate
  • r is the polar diameter
  • i is the imaginary unit
  • D is a constant greater than zero
  • circ(r/R) is the circle domain function
  • R is the radius of the compound spiral phase plate.
  • the thickness distribution h on the polar coordinate of the composite helical phase plate satisfies the following relationship:
  • ⁇ is the wavelength of the modulated laser
  • n 1 is the refractive index of the photosensitive adhesive after curing
  • n 2 is the refractive index of the medium surrounding the composite helical phase plate
  • p is the topological charge
  • f is the composite Design focal length of type helical phase plate
  • r is the polar diameter
  • i is an imaginary unit
  • D is a constant greater than zero.
  • the composite spiral phase plate is provided with groove structures, and the groove depth h 0 of the groove structure satisfies the following relationship:
  • ⁇ is the wavelength of the modulated laser
  • n 1 is the refractive index of the composite helical phase plate
  • n 2 is the refractive index of the medium surrounding the composite helical phase plate.
  • the diameter of the composite helical phase plate is larger than the effective diameter of the multi-mode optical fiber and smaller than the outer diameter of the multi-mode optical fiber.
  • the focused vortex light generator further includes a matching layer disposed between the multimode optical fiber and the composite helical phase plate to make the composite helical phase plate strong Adhesive to the multimode optical fiber.
  • a preparation method of a focused vortex light generator comprising: providing a single-mode optical fiber and a multi-mode optical fiber; obtaining structural data of a composite helical phase plate; and welding the output end of the single-mode optical fiber and the input end of the multi-mode optical fiber
  • the composite helical phase plate is prepared at the output end of the multimode optical fiber according to the structure data of the composite helical phase plate.
  • the method for preparing the composite helical phase plate includes: providing photosensitive glue; fixing the multi-mode optical fiber, and contacting the output end of the multi-mode optical fiber with the photosensitive glue; The photosensitive glue, the cured photosensitive glue forms the structure of the composite helical phase plate; the residual photosensitive glue is removed.
  • the method for removing residual photosensitive adhesive includes: immersing the output end of the multimode optical fiber in propylene glycol methyl ether acetate to remove the unexposed photosensitive adhesive; The output end of the multimode optical fiber that has been treated with acetate is put into an isopropanol solution to wash away propylene glycol methyl ether acetate; the output end of the multimode optical fiber is dried with nitrogen.
  • the method for obtaining the structure data of the composite helical phase plate includes: obtaining the wavelength of the light wave applied by the focused vortex light generator, the required focal length, and the effective communication of the multimode optical fiber Light diameter; the phase function of the composite helical phase plate is calculated according to the wavelength and focal length; the thickness distribution data on the polar coordinate of the composite helical phase plate is obtained according to the phase function, and according to the effective light The diameter obtains the boundary diameter of the composite helical phase plate.
  • the method for obtaining the structure data of the composite helical phase plate further includes: discretizing the continuous thickness distribution data on the polar coordinates of the composite helical phase plate.
  • the thickness distribution h on the polar coordinate of the composite helical phase plate satisfies the following relationship:
  • ⁇ is the wavelength of the modulated laser
  • n 1 is the refractive index of the photosensitive adhesive after curing
  • n 2 is the refractive index of the medium surrounding the composite helical phase plate
  • p is the topological charge
  • f is the composite Design focal length of type helical phase plate
  • r is the polar diameter
  • i is an imaginary unit
  • D is a constant greater than zero.
  • FIG. 1 is a schematic diagram explaining the "pitch"
  • FIG. 3 is a schematic diagram of the structure of the focused vortex light generator provided by this application.
  • Figure 4 is a schematic diagram explaining the principle of vortex light generation
  • Figure 5a is an electron microscope image when the topological charge of the compound spiral phase plate is 0;
  • Figure 5b is an electron micrograph when the topological charge of the compound helical phase plate is 1;
  • Figure 5c is an electron micrograph of the composite spiral phase plate when the topological charge is 2;
  • Figure 6 is a schematic diagram of the principle of discrete or stepped modeling of the composite spiral phase plate provided in Figure 3;
  • FIG. 7a is a flow chart of the preparation method used for preparing the focused vortex light generator of this application.
  • Fig. 7b is a schematic structural diagram of a preparation system for preparing a focused vortex light generator according to the present application.
  • FIG. 8 is a schematic structural diagram of a detection system for detecting the vortex light generated by the focused vortex light generator provided by the present application;
  • Fig. 9 is a focal length detection diagram of the focused vortex light generator provided in Fig. 3.
  • Figure 10 shows the interference pattern of vortex light and parallel light with a topological charge of 1
  • Figure 11 shows the interference pattern of vortex light and parallel light with a topological charge of 2.
  • 100 focused vortex light generator; 110, single-mode fiber; 120, graded index fiber; 121, input end; 122, output end; 123, pitch; 124, transmission light; 130, matching layer; 140, fundamental mode Gaussian light; 150, compound spiral phase plate; 151, isophase surface; 160, vortex light;
  • optical fiber clamp 310, optical fiber clamp; 320, cover glass; 330, photosensitive glue; 340, refractive index matching oil; 350, objective lens; 360, scanning galvanometer;
  • This application provides a focused vortex light generator for modulating laser light into vortex light with focusing characteristics, and provides a method for preparing the focused vortex light generator.
  • the application will be more comprehensively described below with reference to the relevant drawings. description of.
  • the graded index optical fiber 120 includes an input end 121 and an output end 122, and the transmission light 124 enters the input end 121 of the graded index optical fiber 120 and exits from the output end 122. Since the refractive index along the radial direction in the graded-index fiber 120 gradually decreases from the center of the circle to the circumference, the graded-index fiber 120 has the ability to change the propagation direction of the transmitted light 124. As shown in Figure 1, when the transmission light 124 is transmitted in the graded index fiber 120, the path is generally sine wave transmission.
  • the "pitch" is defined as when the transmission light 124 transmits a complete sine waveform, the complete The length of the graded index optical fiber 120 corresponding to the sine waveform.
  • the pitch is 123 as shown in the figure.
  • the transmission path of the transmission light 124 in the graded index fiber 120 is generally sinusoidal.
  • a beam of transmission light 124 When a beam of transmission light 124 is transmitted through the graded index fiber 120, it will periodically diverge and focus.
  • a beam of transmission light 124 enters the graded index fiber 120 from the input end 121, the incident point can be regarded as a focal point, and then the transmission light 124 starts to diverge in the graded index fiber 120.
  • the wavefront of the transmitted light 124 is exactly at (2N+1)/4 times the pitch 123, where N is a natural number.
  • FIG. 2 is a schematic diagram of selecting the length of the graded index fiber 120 in the embodiment provided by this application.
  • the transmission light 124 is incident from the input end 121 of the graded index fiber 120, it is regarded as converging into one point, that is, the transmission light 124 is at the focal point.
  • the beam shape is parallel light, and the beam width reaches the maximum value in the graded index fiber 120, which meets the needs of modulating the vortex light.
  • the distance between the input terminal 121 and the output terminal 122 is about a quarter of the pitch
  • the value of the pitch 123 is different; similarly, for the transmission light 124 of the same wavelength, the value of the pitch 123 is different.
  • the corresponding pitch 123 is also different. Therefore, when determining the length of the graded-index optical fiber 120, the specific types of the transmitted light 124 and the graded-index optical fiber 120 need to be considered comprehensively.
  • the outer diameter of the graded index fiber 120 used is 125 ⁇ m, the core diameter is 62.5 ⁇ m, and the effective clear aperture is 30 ⁇ m.
  • the transmission wavelength of the laser is 1550nm (wavelength in vacuum)
  • the quarter pitch of the optical fiber 120 is 250 ⁇ m.
  • the length of the graded index fiber 120 can be slightly smaller or slightly larger than the quarter pitch.
  • the length of the optical fiber 120 is 250 ⁇ m ⁇ 5 ⁇ m, and the use of this range is beneficial to the error tolerance of processing and facilitates the cutting of the graded index optical fiber 120.
  • the focused vortex light generator 100 mainly includes a single-mode fiber 110, a graded-index fiber 120, and a composite spiral phase plate 150.
  • the single-mode fiber 110 is fused to the input end 121 of the graded-index fiber 120.
  • the output terminal 122 is butt-coupled with the composite spiral phase plate 150.
  • the single-mode fiber 110 is a standard single-mode fiber SMF-28, with an outer diameter of 125 ⁇ m, a core diameter of 8.2 ⁇ m, and a corresponding laser vacuum wavelength of 1550 nm.
  • the laser light generated by the laser is coupled into the single-mode optical fiber 110, and is converted into or left a fundamental mode Gaussian light 140 in the single-mode optical fiber 110.
  • the fundamental mode Gaussian light 140 is transmitted in the single-mode fiber 110, it has stronger stability and anti-interference ability, and is suitable for long-distance transmission. Therefore, the single-mode fiber 110 is mainly used before the laser is modulated by the composite spiral phase plate 150. As a transmission medium.
  • the fundamental mode Gaussian light 140 is coupled into the graded-index fiber 120 at the connection end face of the single-mode fiber 110 and the graded-index fiber 120. Since the core diameter of the graded-index fiber 120 is larger than that of the single-mode fiber 110, the fundamental mode Gaussian light The 140 wavefront can be regarded as being at a focal point on the transmission path. The fundamental mode Gaussian light 140 gradually diverges from the focal point to the output end of the graded-index optical fiber 120 and exits at a quarter of the pitch of the graded-index optical fiber 120.
  • the fundamental mode Gaussian light 140 will diverge and expand in the graded-index fiber 120.
  • the beam is collimated, and finally coupled into the composite helical phase plate 150 with parallel light, and modulated by the composite helical phase plate 150 to generate vortex light with focusing characteristics.
  • Vortex light is a beam carrying orbital angular momentum.
  • the center of the vortex beam has a singularity due to an uncertain phase or a sudden change, and the light intensity at the singularity is zero and there is no heating effect.
  • the isophase surface of vortex light is spiral, in other words, the wavefront of vortex light is spiral. Therefore, when the vortex light is generated from a beam of parallel light, it is only necessary to modulate the phases at different polar angles in the beam to make the isophase plane of the beam spiral. Since the frequency of light does not change with the refractive index, at the same time, the number of waves traveled by light in different refractive indexes is the same.
  • ⁇ is the wavelength in vacuum
  • ⁇ ' is the wavelength of the light under a medium with a refractive index of n.
  • the refractive index of the composite helical phase plate 150 is larger than the refractive index of the surrounding medium, and the light on the left travels longer in the composite helical phase plate 150 with a larger refractive index than the light on the right. q, so the isophase plane 151 of the left side light and the right side light is inclined.
  • n 1 is the refractive index of the composite helical phase plate 150
  • n 2 is the refractive index of the surrounding medium of the composite helical phase plate 150, in one of the embodiments, n 2 is air Refractive index.
  • the composite helical phase plate 150 realizes the above-mentioned effect of generating vortex light by opening a groove structure, and h 0 is the groove depth of the groove structure, and
  • Vortex light has a continuous spiral wavefront, and its phase can be expressed as Where i is the imaginary unit and p is the topological charge, For the polar angle.
  • the topological charge p can be an integer, zero or a fraction.
  • the transmittance function of the Fresnel lens is introduced here, and the expression is Where ⁇ is the wavelength of the modulated laser, f is the design focal length of the Fresnel lens, r is the polar diameter, and i is the imaginary unit.
  • the Fresnel lens has a focusing effect on light.
  • the phase expression of the composite spiral phase plate 150 used for modulating the focused vortex light in this application is obtained, which is:
  • Is the transmittance function of the composite spiral phase plate 150 Is the polar coordinate, r is the polar diameter, Is the polar angle, p is the topological charge, ⁇ is the wavelength of the modulated laser, f is the design focal length of the composite spiral phase plate, i is the imaginary unit, D is a constant greater than zero, which represents the period of the conical lens, circ (r/R) is the circular domain function, and R is the radius of the compound spiral phase plate.
  • the phase expression of the composite spiral phase plate 150 contains the expression of the transmittance function of the vortex light And the focal length factor f. Therefore, it can be seen that the composite helical phase plate 150 has the effect of generating vortex light and focusing the vortex light, and the generated vortex light will be focused at a distance f from the composite helical phase plate 150.
  • the period D of the conical lens can be made to be infinite, and the transmittance function expression of the composite spiral phase plate 150 is changed to:
  • the composite helical phase plate 150 is in polar coordinates
  • the distribution formula of the thickness h at is:
  • the thickness distribution of the composite helical phase plate 150 will also be determined, and will not be changed by the diameter and size of the composite helical phase plate 150. Therefore, setting the diameter of the composite helical phase plate 150 is equivalent to cutting a predetermined sufficiently large composite helical phase plate 150.
  • the effective modulation area of 150 is too small, resulting in insufficient vortex light generation and focusing modulation capabilities.
  • the actual effective diameter of the composite helical phase plate 150 is equivalent to the effective diameter of the incident light, that is, the diameter of the light coupled into the composite helical phase plate 150. Since the effective light-passing radius of the single-mode fiber 110 is too small (less than or equal to 8.2 ⁇ m), it is not enough for the composite helical phase plate 150 to modulate the focused vortex light, so the single-mode fiber 110 and the graded index fiber 120 are fusion spliced, and The vortex light is modulated by coupling the compound helical phase plate 150 at the output end of the graded index fiber 120. Among them, the graded index fiber 120 has the effects of beam expansion and collimation.
  • the specific structure of the composite helical phase plate 150 is also different.
  • the graded index fiber 120 can be replaced with an ordinary multi-mode fiber.
  • the fundamental mode Gaussian light 140 enters the multi-mode fiber from the single-mode fiber 110, since the core diameter of the multi-mode fiber is larger than that of the single-mode fiber 110, the fundamental mode The Gaussian light 140 gradually diffuses in the multimode fiber, that is, the multimode fiber can also expand the fundamental mode Gaussian light 140.
  • the expanded fundamental mode Gaussian light 140 can generate a focused vortex through the compound spiral phase plate 150. Spin light.
  • the diameter of the composite helical phase plate 150 is 30 m-125 m, where 30 m is the effective clear aperture of the graded index optical fiber 120, and 125 m is the outer diameter of the graded index optical fiber 120.
  • the diameter of the composite spiral phase plate 150 is 30 ⁇ m, 33 ⁇ m, 38 ⁇ m, 40 ⁇ m, 45 ⁇ m, 50 ⁇ m, 60 ⁇ m, 80 ⁇ m, 90 ⁇ m, 100 ⁇ m, 110 ⁇ m, 120 ⁇ m, 125 ⁇ m.
  • the diameter of the composite helical phase plate 150 is 69 ⁇ m.
  • the focused vortex light generator 100 further includes a matching layer 130.
  • the matching layer 130 is located between the graded index fiber 120 and the compound helical phase plate 150 and is coaxial with the graded index fiber 120. Please refer to image 3.
  • the matching layer 130 is a uniform refractive index medium. More preferably, the refractive index of the matching layer 130 may be equal to the refractive index of the composite helical phase plate 150, and the material is a cured photosensitive adhesive.
  • the function of the matching layer 130 is to make the composite helical phase plate 150 adhere firmly to the graded index optical fiber 120.
  • the matching layer 130 has a diameter of 30 ⁇ m-125 ⁇ m and a thickness of 0.01 ⁇ m-10 ⁇ m, where 30 ⁇ m is the effective light-passing radius of the graded index optical fiber 120, and 125 ⁇ m is the outer diameter of the graded index optical fiber 120.
  • the diameter of the matching layer 130 is 30 ⁇ m, 33 ⁇ m, 38 ⁇ m, 40 ⁇ m, 45 ⁇ m, 50 ⁇ m, 60 ⁇ m, 80 ⁇ m, 90 ⁇ m, 100 ⁇ m, 110 ⁇ m, 120 ⁇ m, 125 ⁇ m; the thickness is 0.01 ⁇ m, 0.05 ⁇ m, 0.18 ⁇ m, 0.5 ⁇ m, 1 ⁇ m, 2 ⁇ m, 3 ⁇ m, 4 ⁇ m, 5 ⁇ m, 6 ⁇ m, 7 ⁇ m, 8 ⁇ m, 9 ⁇ m, 10 ⁇ m. It should be noted that, under the premise that the composite helical phase plate 150 and the graded index optical fiber 120 are tightly connected, the thinner the matching layer 130 is, the better. Specifically, in one embodiment, the diameter of the matching layer 130 is 70 ⁇ m and the thickness is 5 ⁇ m.
  • the vortex beam generated on the basis of the above embodiment has a bright ring radius at the center of which is proportional to the topological charge. Therefore, a large topological charge produces a larger bright ring radius of the vortex beam.
  • This property makes the vortex light in some Application occasions, such as the measurement of vortex light properties, provide convenience, but in other occasions, such as vortex beam coupling, transmission and other applications, a better method is to keep the radius of the bright ring at the center of the vortex light unchanged.
  • a composite helical phase plate 150 is used to obtain a vortex beam with a constant central bright ring radius.
  • the transmittance function of the conical lens in the composite spiral phase plate 150 is expressed as So the integrated circle function
  • the post transmittance function is expressed as:
  • D is the period of the conical lens, It is a circular domain function, and R is the radius of the composite spiral phase plate 150, and the other parameters are the same as those in the above-mentioned embodiment, and will not be repeated.
  • the thickness distribution h function of the new composite helical phase plate can be calculated as:
  • the vortex light whose central bright ring radius does not change with the topological charge p can be modulated.
  • the diameter of the composite helical phase plate 150 is slightly smaller than the diameter of the matching layer 130.
  • laser ablation is performed on the core of the graded index optical fiber 120 to form the structure of the composite helical phase plate 150.
  • the structure of the composite helical phase plate 150 is prepared on the core of the graded index optical fiber 120 by electron beam lithography technology.
  • the structure of the composite helical phase plate 150 can also be prepared by femtosecond laser two-photon polymerization technology.
  • the method includes:
  • S10 Provide single-mode fiber 110 and graded index fiber 120;
  • the parameters of the single-mode optical fiber 110 and the graded index optical fiber 120 are in accordance with the parameters in the above-mentioned embodiment, and will not be repeated here.
  • the graded index fiber 120 is replaced with a multimode fiber.
  • the structure data of the composite helical phase plate 150 and the matching layer 130 jointly participate in the modeling, wherein the diameter of the matching layer 130 is 70 ⁇ m and the thickness is 5 ⁇ m.
  • the diameter of the composite spiral phase plate 150 is 69 ⁇ m, and the thickness distribution h satisfies:
  • the above-mentioned continuous distribution is discretized, and the discrete data is stepped into a solid model.
  • discretization and stepping can be completed together, and the rules are:
  • the discrete number M is 1, so the thickness distribution of the composite helical phase plate 150 only has two levels, and the value is only two values, which are the maximum value and the minimum value.
  • the dashed line 152 is the smooth thickness distribution
  • the dashed line 154 is the zero position reference line.
  • the smooth thickness distribution value is greater than the zero position reference line, the maximum value is taken.
  • the smooth thickness distribution value is less than the zero position reference line Take the minimum value when it is line.
  • the cross-sectional shape of the composite helical phase plate 150 has a square wave shape as shown in FIG. 6.
  • the foregoing continuous thickness distribution is discretized, and the discrete data is stepped into a solid model.
  • the rules are:
  • the thickness difference on the composite helical phase plate 150 is
  • the discrete number M is an integer greater than or equal to 2, and the larger the discrete number M is, the structure of the composite helical phase plate 150 is closer to a perfect composite helical phase plate, and the vortex light modulated is more perfect.
  • S30 Weld the output end of the single-mode optical fiber 110 and the input end of the graded-index optical fiber 120; couple the light energy in the single-mode optical fiber 110 into the graded-index optical fiber 120.
  • FIG. 7b is a schematic diagram of the preparation system of this embodiment, which includes an optical fiber clamp 310, a cover glass 320, a photosensitive glue 330, a refractive index matching oil 340, an objective lens 350, and a scanning galvanometer 360.
  • the photosensitive glue 330 is selected from IP- L780.
  • the femtosecond laser used for printing sequentially passes through the scanning galvanometer 360 for controlling the direction of the femtosecond laser, is focused by the objective lens 350, and then irradiates the photosensitive adhesive 330 through the refractive index matching oil 340 and the cover glass 320, and the optical fiber clamp 310 Used to clamp the focused vortex light generator 100 to be processed.
  • the focused vortex light generator 100 to be processed on the optical fiber clamp 310, which can adjust the distance between the end face of the optical fiber and the cover glass 320 with micrometer precision.
  • the thickness of the cover glass 320 is 170 ⁇ m
  • the upper and lower surfaces of the cover glass 320 are respectively deposited with photosensitive glue 330 and refractive index matching oil 340
  • the output end of the objective lens 350 is connected with the refractive index matching oil 340.
  • the optical fiber clamp 310 is fixed on a three-dimensional scanning pressure level translation stage (not shown in the figure) controlled by a controller (not shown in the figure). Adjusting the piezoelectric stage can accurately position the focal point of the femtosecond laser at the center of the output end 122 of the graded index fiber 120.
  • the structure data of the composite helical phase plate 150 is input to the controller, and the 3D printing operation is performed to write the specific structure of the composite helical phase plate 150; then, the semi-finished product of the focused vortex light generator 100 after writing has a composite
  • the end of the helical phase film 150 is immersed in a propylene glycol methyl ether acetate bath for 8 minutes to 35 minutes to remove the unexposed residual photosensitive adhesive 330; secondly, it is placed in a solution containing isopropanol for 1 minute to 5 minutes Minutes; finally, dry the end of the composite helical phase plate 150 very gently with nitrogen to form the focused vortex light generator 100.
  • propylene glycol methyl ether acetate will not affect the cured photosensitive adhesive 330, so the end of the composite spiral phase film 150 can actually be immersed in the propylene glycol methyl ether acetate bath for longer than 35 minutes. .
  • the isopropanol solution treatment time can also exceed 5 minutes.
  • it has a better processing effect and avoids wasting too much time.
  • the treatment time of propylene glycol methyl ether acetate is 30 minutes; the treatment time of the isopropanol solution is 2 minutes.
  • the unexposed photosensitive adhesive 330 is dissolved in propylene glycol methyl ether acetate, while propylene glycol methyl ether acetate is easy to dissolve in the isopropanol solution. Since isopropanol is easy to volatilize, it can affect the residual propylene glycol methyl ether acetate. Cleaning effect.
  • Nitrogen is a chemically stable and easy-to-obtain gas. The use of nitrogen can dry the end of the composite helical phase film 150 while not reacting with isopropanol, cured photosensitive adhesive 330 and other related substances. Play the role of protecting the focused vortex light generator 100.
  • the interference detection system shown in FIG. 8 which includes a laser 410, a coupler 420, a micro-displacement platform 430, an objective lens 440, a mirror 450, a beam splitter 460 (the beam combining function is used in this embodiment), and an image Sensor 470.
  • the laser light generated by the laser 410 is coupled into the focused vortex light generator 100 and the ordinary single-mode fiber 200 through the coupler 420, and the laser light passes through the focused vortex light generator 100 and modulates the vortex light 160, which is output after the ordinary single-mode fiber 200 Parallel light 210.
  • the focused vortex light generator 100 and the ordinary single-mode optical fiber 200 are respectively adjusted to the objective lens 440 through the micro-displacement platform 430, so that the laser is coupled into the objective lens 440 and expands, and the expanded two beams are combined by the beam splitter 460 When sent to the image sensor 470, the vortex light 160 interferes with the parallel light 210 to form an interference image.
  • the interference detection system also includes a diaphragm 480.
  • the diaphragm 480 blocks the light beam of the ordinary single-mode optical fiber 200, the focused transmission pattern of the vortex light 160 can be directly observed through the image sensor 470. Moving the micro-displacement platform 430 along the light propagation direction can clearly observe the focus intensity distribution images of the vortex light 160 at the front and back positions near the designed focus to determine whether the vortex light 160 is successfully focused, and to detect the actual focus point position .
  • the reflector 450 plays the role of adjusting the optical path.
  • the laser 410 is a tunable laser to generate laser light with a wavelength suitable for the design wavelength of the focused vortex light generator 100.
  • the objective lens 440 is a 40 ⁇ lens, and the image sensor 470 is an infrared CCD (Charge Coupled Device). Coupling device) sensor.
  • CCD Charge Coupled Device
  • the actual focal length and the designed focal length of 20 ⁇ m have a certain deviation, and the magnitude of the deviation can be predicted by simulation calculation.
  • the interference image of the vortex light with the topological charge p of 1 is shown in Fig. 10, and the interference fringe in Fig. 10 has a dislocation, as shown at the position P1 in Fig. 10.
  • the interference image of the vortex light with the topological charge p of 2 is shown in Fig. 11.
  • Fig. 11 there are two dislocations of interference fringes, as shown at P2 in Fig. 11. It can be understood that the number of dislocations in the interference image is equal to the topological charge p of the vortex light.
  • the topological charge of the focused vortex light generated by the focused vortex light generator 100 can be detected and determined.

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Abstract

一种聚焦涡旋光发生器及其制备方法，其中聚焦涡旋光发生器包括：单模光纤(110)、渐变折射率光纤(120)和螺旋波带片(150)，渐变折射率光纤(120)的输入端与单模光纤(110)对接耦合，渐变折射率光纤(120)的输出端与复合型螺旋相位片(150)对接耦合。

Description

聚焦涡旋光发生器及其制备方法 技术领域
本发明涉及衍射光学领域，特别是涉及一种聚焦涡旋光发生器及其制备方法。
背景技术
涡旋光束由于其独特的螺旋波前结构和中心奇点特性而在通信、传感以及粒子操控等领域获得了广泛应用。以往涡旋光束的产生依赖于自由空间光耦合，如圆柱-透镜模式转换器，空间光调制器，微观环形谐振器和螺旋相位板等。
但是，涡旋光束在传播过程中具有高发散特性，使得其在如涡旋光耦合、涡旋光微粒操控等一些实际应用中并不为人们所期望。传统的涡旋光束发生器件虽能够产生出涡旋光束，但是并不具备同时对所产生涡旋光束的聚焦能力。因此，在应用中需要借助外在的光学元件来对涡旋光束进行聚焦。外在聚焦元件的引入不仅增大了整个光学系统的体积，增添了繁琐的光束准直过程，且降低了系统的稳定性。
发明内容
根据本申请的各种实施例，提供一种聚焦涡旋光发生器及其制备方法。
为了达到上述技术效果，本申请采用如下技术方案：
一种聚焦涡旋光发生器，包括：单模光纤、多模光纤和复合型螺旋相位片，所述多模光纤的输入端与所述单模光纤对接耦合，所述多模光纤的输出 端与所述复合型螺旋相位片对接耦合。
在其中一些实施例中，所述多模光纤为渐变折射率光纤，所述渐变折射率光纤的长度为节距的(2N+1)/4倍，其中N为自然数。
在其中一些实施例中，所述复合型螺旋相位片的透过率函数
Figure PCTCN2020077903-appb-000001
满足如下关系：
Figure PCTCN2020077903-appb-000002
其中p为拓扑荷数，λ为被调制激光的波长，f为所述复合型螺旋相位片的设计焦距，
Figure PCTCN2020077903-appb-000003
为极坐标，r为极径，
Figure PCTCN2020077903-appb-000004
为极角，i为虚数单位，D为大于零的常数，circ(r/R)为圆域函数，R为复合型螺旋相位片的半径。
在其中一些实施例中，所述复合型螺旋相位片的极坐标上厚度分布h满足如下关系：
Figure PCTCN2020077903-appb-000005
其中，
Figure PCTCN2020077903-appb-000006
其中，λ为被调制激光的波长，n 1为固化后所述光敏胶的折射率，n 2为所述复合型螺旋相位片周围介质的折射率，p为拓扑荷数，f为所述复合型螺旋相位片的设计焦距，
Figure PCTCN2020077903-appb-000007
为极坐标，r为极径，
Figure PCTCN2020077903-appb-000008
为极角，i为虚数单位，D为大于零的常数。
在其中一些实施例中，所述复合型螺旋相位片分布有槽体结构，所述槽体结构的槽深h 0满足如下关系：
Figure PCTCN2020077903-appb-000009
其中，λ为被调制激光的波长，n 1为所述复合型螺旋相位片的折射率，n 2 为所述复合型螺旋相位片周围介质的折射率。
在其中一些实施例中，所述复合型螺旋相位片的直径大于所述多模光纤的有效通光直径，且小于所述多模光纤的外径。
在其中一些实施例中，所述聚焦涡旋光发生器还包括匹配层，所述匹配层设置于所述多模光纤与所述复合型螺旋相位片之间，使所述复合型螺旋相位片牢固的粘附在所述多模光纤上。
一种聚焦涡旋光发生器的制备方法，包括:提供单模光纤和多模光纤；获取复合型螺旋相位片的结构数据；熔接所述单模光纤的输出端和所述多模光纤的输入端；依据所述复合型螺旋相位片的结构数据在所述多模光纤的输出端制备所述复合型螺旋相位片。
在其中一些实施例中，制备所述复合型螺旋相位片的方法包括：提供光敏胶；固定所述多模光纤，并使所述多模光纤的输出端接触所述光敏胶；使用激光固化所述光敏胶，固化后的所述光敏胶形成所述复合型螺旋相位片的结构；去除残余的光敏胶。
在其中一些实施例中，所述去除残余的光敏胶的方法包括：将所述多模光纤的输出端浸入丙二醇甲醚乙酸酯中以除去未曝光的所述光敏胶；将经丙二醇甲醚乙酸酯处理过的所述多模光纤的输出端放入异丙醇溶液中处理，以洗去丙二醇甲醚乙酸酯；用氮气吹干所述多模光纤的输出端。
在其中一些实施例中，所述获取复合型螺旋相位片的结构数据的方法包括：获取所述聚焦涡旋光发生器所应用的光波波长，所需要的焦距，以及所述多模光纤的有效通光直径；依据所述波长和焦距计算得到所述复合型螺旋相位片的相位函数；依据所述相位函数得到所述复合型螺旋相位片的极坐标上厚度分布数据，以及依据所述有效通光直径得到所述复合型螺旋相位片的 边界直径。
在其中一些实施例中，所述获取复合型螺旋相位片的结构数据的方法还包括：对所述复合型螺旋相位片的极坐标上连续的厚度分布数据进行离散化处理。
在其中一些实施例中，所述复合型螺旋相位片的极坐标上厚度分布h满足如下关系：
Figure PCTCN2020077903-appb-000010
其中，
Figure PCTCN2020077903-appb-000011
其中，λ为被调制激光的波长，n 1为固化后所述光敏胶的折射率，n 2为所述复合型螺旋相位片周围介质的折射率，p为拓扑荷数，f为所述复合型螺旋相位片的设计焦距，
Figure PCTCN2020077903-appb-000012
为极坐标，r为极径，
Figure PCTCN2020077903-appb-000013
为极角，i为虚数单位，其中D为大于零的常数。
本发明的一个或多个实施例的细节在下面的附图和描述中提出。本发明的其他特征、目的和优点将从说明书、附图以及权力要求书变得明显。
附图说明
为了更好地描述和说明这里公开的那些发明的实施例和/或示例，可以参考一幅或多幅附图。用于描述附图的附加细节或示例不应当被认为是对所公开的发明、目前描述的实施例和/或示例以及目前理解的这些发明的最佳模式中的任何一者的范围的限制。
图1为解释“节距”的示意图；
图2为本申请提供的渐变折射率光纤的长度与节距关系的示意图；
图3为本申请提供的聚焦涡旋光发生器的结构示意图；
图4为解释涡旋光产生的原理示意图；
图5a为复合型螺旋相位片拓扑荷数为0时的电镜图；
图5b为复合型螺旋相位片拓扑荷数为1时的电镜图；
图5c为复合型螺旋相位片拓扑荷数为2时的电镜图；
图6为对图3中提供的复合型螺旋相位片离散或阶梯化建模的原理示意图；
图7a为本申请用于制备聚焦涡旋光发生器的制备方法流程图；
图7b为本申请用于制备聚焦涡旋光发生器的制备系统结构示意图；
图8为用于检测本申请提供的聚焦涡旋光发生器产生涡旋光的检测系统的结构示意图；
图9为图3中提供的聚焦涡旋光发生器焦距检测图；
图10为拓扑荷数为1的涡旋光与平行光的干涉图样；
图11为拓扑荷数为2的涡旋光与平行光的干涉图样。
附图标记：
100，聚焦涡旋光发生器；110，单模光纤；120，渐变折射率光纤；121，输入端；122，输出端；123，节距；124，传输光；130，匹配层；140，基模高斯光；150，复合型螺旋相位片；151，等相位面；160，涡旋光；
200，普通单模光纤；210，平行光；
310，光纤夹具；320，盖玻片；330，光敏胶；340，折射率匹配油；350，物镜；360，扫描振镜；
410，激光器；420，耦合器；430，微位移平台；440，物镜；450，反射镜；460，分束器；470，图像传感器；480，光阑。
具体实施方式
附图中给出了本申请的首选实施例。但是，本申请可以以许多不同的形式来实现，并不限于本文所描述的实施例。相反地，提供这些实施例的目的是使本申请的公开内容更加透彻全面。
需要说明的是，当元件被称为“固定”于另一个元件，它可以直接在另一个元件上或者也可以存在居中的元件。当一个元件被认为是“连接”另一个元件，它可以是直接连接到另一个元件或者可能同时存在居中元件。相反，当元件被称作“直接在”另一元件“上”时，不存在中间元件。本文所使用的术语“垂直的”、“水平的”、“左”、“右”以及类似的方位或位置表述只是为了说明的目的。
除非另有定义，本文所使用的所有的技术和科学术语与属于本申请的技术领域的技术人员通常理解的含义相同。本文说明书中所使用的术语只是为了描述具体的实施例的目的，不是旨在于限制本申请。本文所使用的术语“及/或”包括一个或多个相关的所列项目的任意的和所有的组合。
本申请提供了一种聚焦涡旋光发生器，用于将激光调制为具有聚焦特性的涡旋光，并提供了一种聚焦涡旋光发生器的制备方法，下面将参照相关附图对本申请进行更全面的描述。
下面对文中所述“节距”一词进行解释。请参照图1，渐变折射率光纤120包括输入端121和输出端122，传输光124从渐变折射率光纤120的输入端121入射，并从输出端122射出。由于渐变折射率光纤120内沿径向的折射率从圆心到圆周处逐渐减小，所以渐变折射率光纤120具备改变传输光124传播方向的能力。依照图1所示意的，传输光124在渐变折射率光纤120内传输时，路径大体呈正弦波形传输，在此将“节距”定义为，传输光124传输一个完整的正弦波形时，该完整的正弦波形所对应的渐变折射率光纤120 的长度。如图中所示的节距123。
由图1可知，传输光124在渐变折射率光纤120内的传输路径大体呈正弦波形，当一束传输光124在渐变折射率光纤120传输时，会周期性的发散和聚焦。如图中所示，一束传输光124从输入端121射入渐变折射率光纤120，入射点可以看做一个聚焦点，然后传输光124在渐变折射率光纤120开始发散。当发散度达到最大时，传输光124的波前正好位于(2N+1)/4倍的节距123处，其中N为自然数。如在图中所示的四分之一倍节距处、四分之三倍节距处、四分之五倍节距处、以及四分之七倍节距处，传输光124的发散度达到最大。若这时传输光124从输出端122射出时，出射的传输光124的波前为平面，如图中所示，出射光为平行光，具有良好的相干性且易于聚焦。
传输光124在光纤中尤其是多模光纤中传播时，同时伴随着光损耗、外界光干扰以及光信号稳定性变差等问题，因此尽可能的缩短光纤的长度有助于减轻上述问题带来的不良影响。图2为本申请提供的实施例中，选择渐变折射率光纤120长度的示意图，传输光124从渐变折射率光纤120的输入端121入射时看作汇聚为一个点，即传输光124处于焦点处，当传输光124于输出端122出射时，光束形状为平行光，且光束的束宽达到在渐变折射率光纤120中的最大值，满足调制涡旋光的需要，这时渐变折射率光纤120的输入端121至输出端122之间的距离约为四分之一倍节距，
可以理解的是，对于不同波长的传输光124，在同一种渐变折射率光纤120中传输时，其节距123的值存在差异；同理，对于同一波长的传输光124，在不同折射率分布的渐变折射率光纤120中传输时，所对应的节距123也不同。因此，确定渐变折射率光纤120的长度时，需结合传输光124与渐变折射率光纤120的具体类型来综合考虑。
本申请的实施例中，采用的渐变折射率光纤120的外径为125μm，芯径为62.5μm，有效通光孔径为30μm，当激光的传输波长为1550nm(真空中波长)时，渐变折射率光纤120的四分之一节距为250μm。鉴于传输光124在四分之一节距处波前的曲率非常小，事实上渐变折射率光纤120的长度可以略小于或略大于四分之一节距，如本实施例中，渐变折射率光纤120的长度为250μm±5μm，采用该范围有利于在加工的容错度内，方便对渐变折射率光纤120进行切割。
下面将参照相关附图对本申请的实施例进行详细描述。
请参照图3，为本申请提供的聚焦涡旋光发生器的结构示意图。其中，聚焦涡旋光发生器100主要包括单模光纤110、渐变折射率光纤120以及复合型螺旋相位片150，单模光纤110熔接在渐变折射率光纤120的输入端121，渐变折射率光纤120的输出端122与复合型螺旋相位片150对接耦合。
在其中一个实施例中，单模光纤110为标准单模光纤SMF-28，其外径为125μm，芯径为8.2μm，对应的激光真空中波长为1550nm。
激光器产生的激光耦合入单模光纤110中，并在单模光纤110中转化为或留下基模高斯光140。当基模高斯光140在单模光纤110中传输时，具有更强的稳定性和抗干扰能力，适合远距离传输，因此在激光被复合型螺旋相位片150调制之前，主要采用单模光纤110作为传输介质。
基模高斯光140在单模光纤110与渐变折射率光纤120的连接端面耦合进入渐变折射率光纤120中，由于渐变折射率光纤120的芯径大于单模光纤110的芯径，基模高斯光140的波前可以视作处于传输路径上的一个焦点处。从该焦点处到渐变折射率光纤120的输出端基模高斯光140逐渐发散，并在渐变折射率光纤120中四分之一倍的节距处出射。鉴于上述对渐变折射率光 纤120中光的传播规律的描述，当渐变折射率光纤120的长度正好为四分之一倍节距时，基模高斯光140会在渐变折射率光纤120中发散扩束且被准直，最后以平行光耦合入复合型螺旋相位片150，经由复合型螺旋相位片150调制后生成具有聚焦特性的涡旋光。
涡旋光为携带轨道角动量的光束，在传输过程中涡旋光束中心因相位不确定或发生突变而产生奇点，且在奇点处的光强为零、无加热效应。直观上理解，涡旋光的等相位面为螺旋状，换而言之，涡旋光的波前为螺旋状。因此，当由一束平行光生成涡旋光时，仅需对光束中不同极角上的相位进行调制，即可使光束的等相位面呈螺旋状。由于光的频率不随折射率改变，在相同的时间里，处于不同折射率中的光走过的波数是相同的。由λ＝nλ′可知，在相同的时间里，处于大折射率介质下的光走过的距离较短，处于小折射率介质下的光走过的距离较长。其中，λ为真空中波长，λ′为该光处于折射率为n的介质下的波长。换言之，当一束平行光其中一部分穿过大折射率介质，而另一部分穿过小折射率介质时，该光束中穿过大折射率介质部分的波前落后于穿过小折射率介质部分的波前。如图4所示，复合型螺旋相位片150的折射率相较周围介质的折射率较大，且左侧光线在折射率较大的复合型螺旋相位片150中传输的距离比右侧光线大q，因此左侧光线与右侧光线的等相位面151发生倾斜。
基于上述原理，为了使平行光通过介质生成连续的涡旋光，需使复合型螺旋相位片150的最低处(或称最薄处)与最高处(或称最厚处)之间的距离h 1满足如下关系：
Figure PCTCN2020077903-appb-000014
其中，λ为被调制激光的波长，n 1为复合型螺旋相位片150的折射率，n 2为复合型螺旋相位片150的周围介质的折射率，在其中一个实施例中，n 2为空气折射率。
在其中一个实施例中，复合型螺旋相位片150通过开设槽体结构实现上述产生涡旋光的效果，h 0即为槽体结构的槽深，且
Figure PCTCN2020077903-appb-000015
涡旋光具有连续的螺旋状波前，其相位可表达为
Figure PCTCN2020077903-appb-000016
其中i为虚数单位，p为拓扑荷数，
Figure PCTCN2020077903-appb-000017
为极角。拓扑荷数p可以为整数，也可以为零或者分数，拓扑荷数p决定最终调制出的涡旋光的形状，如拓扑荷数p＝0时，涡旋光的形状为同心圆环状，当拓扑荷数p＝1时，涡旋光的形状为单螺旋状，当拓扑荷数p＝2时，涡旋光的形状为双螺旋状，涡旋光的形状可以参考图9中涡旋光聚焦时的形状(每一行最后的图案)。
为了使涡旋光聚焦，这里引入菲涅尔透镜的透过率函数，表达式为
Figure PCTCN2020077903-appb-000018
其中λ为被调制激光的波长，f为菲涅尔透镜的设计焦距，r为极径，i为虚数单位。菲涅尔透镜具有对光的聚焦效果。
为了得到中心亮环半径不变的涡旋光束，同时引入锥透镜的透过率函数，表达式为
Figure PCTCN2020077903-appb-000019
其中D为大于零的常数，表征锥透镜的周期。
通过普通螺旋相位片、菲涅尔透镜以及锥透镜的透过率函数相乘，得到本申请中用于调制聚焦涡旋光的元器件复合型螺旋相位片150的相位表达式，为：
Figure PCTCN2020077903-appb-000020
其中，
Figure PCTCN2020077903-appb-000021
为复合型螺旋相位片150的透过率函数，
Figure PCTCN2020077903-appb-000022
为极坐标，r为极径，
Figure PCTCN2020077903-appb-000023
为极角，p为拓扑荷数，λ为被调制激光的波长，f为所述复 合型螺旋相位片的设计焦距，i为虚数单位，D为大于零的常数，表征锥透镜的周期，circ(r/R)为圆域函数，R为复合型螺旋相位片的半径。从式中看出，复合型螺旋相位片150的相位表示式中含有涡旋光的透过率函数表达式
Figure PCTCN2020077903-appb-000024
以及焦距因子f。因此可知，复合型螺旋相位片150具有产生涡旋光并对涡旋光聚焦的效果，且产生的涡旋光会在距复合型螺旋相位片150长度为f处聚焦。
仅考虑对聚焦涡旋光发生器的聚焦能力时，可以使锥透镜的周期D趋于无穷大，则复合型螺旋相位片150的透过率函数表达式变更为：
Figure PCTCN2020077903-appb-000025
通过进一步计算，可以得出复合型螺旋相位片150于极坐标
Figure PCTCN2020077903-appb-000026
处的厚度h的分布公式为：
Figure PCTCN2020077903-appb-000027
其中，
Figure PCTCN2020077903-appb-000028
其中，
Figure PCTCN2020077903-appb-000029
以及，
Figure PCTCN2020077903-appb-000030
其中各参数的含义不再赘述。
由上式可以看出，当式中各参数确定后，复合型螺旋相位片150的厚度分布也将确定，且不以复合型螺旋相位片150直径尺寸的改变而改变。因此， 设定复合型螺旋相位片150的直径，相当于对一片既定的足够大的复合型螺旋相位片150进行切割。复合型螺旋相位片150的直径越大，复合型螺旋相位片150的结构更加完整，调制出的涡旋光的效果越好，而当复合型螺旋相位片150的直径过小时，复合型螺旋相位片150的有效调制面积过小，导致涡旋光产生和聚焦调制的能力不足。而复合型螺旋相位片150的实际有效直径相当于入射光线的有效直径，也就是耦合入复合型螺旋相位片150的光线的直径。由于单模光纤110的有效通光半径过小(小于或等于8.2μm)，不足以使复合型螺旋相位片150调制出聚焦涡旋光，故采用单模光纤110与渐变折射率光纤120熔接，且于渐变折射率光纤120的输出端对接耦合复合型螺旋相位片150的方式调制涡旋光。其中，渐变折射率光纤120起到扩束和准直的效果。
上述实施例中，对于调制拓扑荷数p不同的涡旋光，复合型螺旋相位片150的具体结构也不一样。在其中一个实施例中，请参照图5a，为拓扑荷数p＝0时复合型螺旋相位片150的结构形状，其为多个同心圆槽体的结构，结构参数与菲涅尔透镜一致。请参照图5b，为拓扑荷数p＝1时复合型螺旋相位片150的结构形状，其槽体呈单螺旋结构。请参照图5c，为拓扑荷数p＝2时复合型螺旋相位片150的结构形状，其槽体呈双螺旋结构。
渐变折射率光纤120可以替换为普通多模光纤，当基模高斯光140从单模光纤110进入到多模光纤中时，由于多模光纤的芯径大于单模光纤110的芯径，基模高斯光140在多模光纤中逐渐扩散，即多模光纤同样能对基模高斯光140起到扩束的效果，扩束后的基模高斯光140通过复合型螺旋相位片150能生成聚焦涡旋光。
复合型螺旋相位片150的直径为30μm-125μm，其中30μm为渐变折射率 光纤120的有效通光孔径，125μm为渐变折射率光纤120的外径。如，复合型螺旋相位片150的直径为30μm，33μm，38μm，40μm，45μm，50μm，60μm，80μm，90μm，100μm，110μm，120μm，125μm。具体的，在一个实施例中，复合型螺旋相位片150的直径为69μm。
在其中一个实施例中，聚焦涡旋光发生器100还包括匹配层130，匹配层130位于渐变折射率光纤120与复合型螺旋相位片150之间，且与渐变折射率光纤120同轴，请参照图3。匹配层130为均匀折射率介质，更佳的，匹配层130的折射率可以等于复合型螺旋相位片150的折射率，材质为固化后的光敏胶。匹配层130的作用为使复合型螺旋相位片150牢固的粘附在渐变折射率光纤120上。可选的，匹配层130的直径为30μm-125μm，厚度为0.01μm-10μm，其中30μm为渐变折射率光纤120的有效通光半径，125μm为渐变折射率光纤120的外径。如，匹配层130的直径为30μm，33μm，38μm，40μm，45μm，50μm，60μm，80μm，90μm，100μm，110μm，120μm，125μm；厚度为0.01μm，0.05μm，0.18μm，0.5μm，1μm，2μm，3μm，4μm，5μm，6μm，7μm，8μm，9μm，10μm。应当说明的是，当满足复合型螺旋相位片150与渐变折射率光纤120紧密连接的前提下，匹配层130越薄越好。具体的，在一个实施例中，匹配层130的直径为70μm，厚度为5μm。
在上述实施例的基础上产生的涡旋光束，其中心亮环半径与拓扑荷值成正比，因此大的拓扑荷值产生涡旋光束的亮环半径也较大，该性质使得涡旋光在一些应用场合，如涡旋光属性的测量上提供了便利，但在另一些场合，如涡旋光束的耦合、传输等应用场合，更佳的手段为保持涡旋光中心亮环半径不变。
因此，在上述实施例的基础上，本申请针对上述应用问题还提供一个具 体的实施例，本实施例利用复合型螺旋相位片150得到中心亮环半径不变的涡旋光束。其中复合型螺旋相位片150中锥透镜透过率函数表达为
Figure PCTCN2020077903-appb-000031
因此综合圆域函数
Figure PCTCN2020077903-appb-000032
后透过率函数表达为：
Figure PCTCN2020077903-appb-000033
其中，D为锥透镜的周期，
Figure PCTCN2020077903-appb-000034
为圆域函数，R为复合型螺旋相位片150的半径，其余参数与上述实施例的相同，不再赘述。
相应的，可以计算得到新的复合型螺旋相位片的厚度分布h函数为：
Figure PCTCN2020077903-appb-000035
其中，
Figure PCTCN2020077903-appb-000036
以及，
Figure PCTCN2020077903-appb-000037
参数含义不再赘述。
当复合型螺旋波带片150的厚度分布h满足上述关系式时，能调制出中心亮环半径不随拓扑荷数p变化的涡旋光。
在其中一个实施例中，为了便于制备复合型螺旋相位片150，复合型螺旋相位片150的直径略小于匹配层130的直径。
在其中一个实施例中，通过激光在渐变折射率光纤120的纤芯上进行烧蚀，进而形成复合型螺旋相位片150的结构。
在其中一个实施例中，通过电子束光刻技术在渐变折射率光纤120的纤芯上制备形成复合型螺旋相位片150的结构。
复合型螺旋相位片150的结构也可以通过飞秒激光双光子聚合技术进行制备。
请参照图7a，其方法包括：
S10：提供单模光纤110和渐变折射率光纤120；
单模光纤110和渐变折射率光纤120的参数依照上述实施例中的参数，此处不再赘述。在其中一个实施例中渐变折射率光纤120替换为多模光纤。
S20：获取复合型螺旋相位片150的结构数据；
在一个实施例中，复合型螺旋相位片150的结构数据与匹配层130共同参与建模，其中匹配层130的直径为70μm，厚度为5μm。复合型螺旋相位片150的直径为69μm，厚度分布h满足：
Figure PCTCN2020077903-appb-000038
其中，
Figure PCTCN2020077903-appb-000039
以及，
Figure PCTCN2020077903-appb-000040
其中各参数的含义不再赘述。上述h的值域为(0，2h 0]。
在一个实施例中，为了简化制备方式、降低生产成本，对上述连续的分布离散化，并将离散数据阶梯化为实体模型。实际建模结构数据时，离散化与阶梯化可以一起完成，其规则为：
S21：确定离散数M。
S22：阶梯化，按
Figure PCTCN2020077903-appb-000041
其中M为离散数，h′为阶梯化后的厚度分布，即3D打印时采用的结构数据，h为光滑的厚度分布，
Figure PCTCN2020077903-appb-000042
y＝{x}为远离零取整函数，如{5.1}＝6，{-2.2}＝-3。
上述实施例中，从0-2h 0离散为2M个数值，阶梯数即为(2M-1)，如希望从0-2h 0离散为10个数值，则令M＝5，这时，阶梯数为9。
在其中一个实施例中，离散数M为1，因此复合型螺旋相位片150的厚度分布只存在两阶，取值仅取两个值，分别为最大值和最小值。如图6所示，虚线152为光滑的厚度分布，虚线154为零位置参考线，当光滑的厚度分布 值大于零位置参考线的厚度时取最大值，当光滑的厚度分布值小于零位置参考线时取最小值。最后，复合型螺旋相位片150的截面形状有如图6所示的方波状。
对应于上述实施例，在另一个实施例中，为了简化制备方式，对上述连续的厚度分布离散化，并将离散数据阶梯化为实体模型。实际建模结构数据时，离散化与阶梯化可以一起完成，其规则为：
S21：确定离散数M。
S22：阶梯化，按
Figure PCTCN2020077903-appb-000043
其中M为离散数，h′为阶梯化后的厚度分布，h为光滑的厚度分布，
Figure PCTCN2020077903-appb-000044
y＝[x]为退一取整函数，又称高斯取整函数，如[5.1]＝5，[-2.2]＝-3。
上述实施例中，从0-2h 0离散为M个数值，阶梯数即为(M-1)，如希望从0-2h 0离散为10个数值，则令M＝10，这时，阶梯数为9。
在上述实施例下，当取离散数M为2时，复合型螺旋相位片150上的厚度差为
Figure PCTCN2020077903-appb-000045
可以理解的是，离散数M取大于等于2的整数，且离散数M越大，复合型螺旋相位片150的结构更接近完美的复合型螺旋相位片，调制出的涡旋光越完美。
S30：熔接单模光纤110的输出端和渐变折射率光纤120的输入端；使单模光纤110中的光能耦合入渐变折射率光纤120中。
S40：依据复合型螺旋相位片150的结构数据在渐变折射率光纤120的输出端制备复合型螺旋相位片150。
请参照图7b，为本实施例的制备系统示意图，其中包括光纤夹具310、 盖玻片320、光敏胶330、折射率匹配油340、物镜350、扫描振镜360，其中光敏胶330选用IP-L780。用于打印的飞秒激光依次通过用于控制飞秒激光方向的扫描振镜360、经物镜350聚焦，再通过折射率匹配油340和盖玻片320后照射到光敏胶330上，光纤夹具310用于夹持待加工的聚焦涡旋光发生器100。
首先，将待加工的聚焦涡旋光发生器100固定于光纤夹具310上，光纤夹具310能够以微米精度调节光纤端面和盖玻片320之间的距离。盖玻片320的厚度为170μm，其上下表面分别沉积了光敏胶330和折射率匹配油340，物镜350的输出端连接折射率匹配油340。用于打印的飞秒激光通过扫描振镜360，再经63×倍、数值孔径NA＝1.4油镜后被聚焦于光敏胶330，并固化光敏胶330。
然后，光纤夹具310被固定在控制器(图中未示出)控制的三维扫描压电平位移台(图中未示出)上。调节压电位移台，可使飞秒激光的聚焦点精确的定位在渐变折射率光纤120输出端122的中心位置。
此外，将复合型螺旋相位片150的结构数据输入控制器，进行3D打印操作，写制复合型螺旋相位片150的具体结构；随后，将写制后的聚焦涡旋光发生器100半成品上具有复合型螺旋相位片150的端部浸入丙二醇甲醚乙酸酯浴中8分钟-35分钟，以除去未曝光的残留光敏胶330；其次，将其放入盛有异丙醇溶液中1分钟-5分钟；最后，用氮气非常轻柔地吹干具有复合型螺旋相位片150的端部，制成聚焦涡旋光发生器100。
应当说明的是，丙二醇甲醚乙酸酯不会影响到固化后的光敏胶330，所以具有复合型螺旋相位片150的端部浸入丙二醇甲醚乙酸酯浴中的时间实际上可以长于35分钟。出于同样的理由，异丙醇溶液处理时间也可以超过5分 钟。但在上述时间范围内时，具有更佳的处理效果，且避免浪费太多时间。
具体的，丙二醇甲醚乙酸酯处理时间为30分钟；异丙醇溶液处理时间为2分钟。
未曝光的光敏胶330溶解于丙二醇甲醚乙酸酯，同时丙二醇甲醚乙酸酯易溶解于异丙醇溶液中，由于异丙醇易于挥发，可对残留的丙二醇甲醚乙酸酯起到清洁作用。氮气是一种化学性质比较稳定，且易于得到的气体，使用氮气可以在吹干具有复合型螺旋相位片150的端部的同时，不与异丙醇、固化的光敏胶330等相关物质反应，起到保护聚焦涡旋光发生器100的作用。
制备聚焦涡旋光发生器100后，需对聚焦涡旋光发生器100的特性进行检测，确保聚焦涡旋光发生器100能准确的调制出聚焦涡旋光。因此采用如图8所示干涉检测系统，其中包括激光器410、耦合器420、微位移平台430、物镜440、反射镜450、分束器460(本实施例中使用其合束功能)，以及图像传感器470。激光器410产生的激光，经耦合器420分别耦合入聚焦涡旋光发生器100和普通单模光纤200，且激光经过聚焦涡旋光发生器100后调制出涡旋光160，经普通单模光纤200后输出平行光210。聚焦涡旋光发生器100和普通单模光纤200分别经微位移平台430调整与物镜440的距离，使激光耦合入物镜440并进行扩束，扩束后的两束光经分束器460合束送入图像传感器470中，涡旋光160与平行光210发生干涉形成干涉图像。
干涉检测系统还包括光阑480，当光阑480遮挡普通单模光纤200的光束时，可以通过图像传感器470直接观测到涡旋光160的聚焦传输图样。沿光的传播方向移动微位移平台430，可清晰地观察到涡旋光160在设计的焦点附近前后位置点上的聚焦强度分布图像，以判断涡旋光160是否成功聚焦，以及检测实际聚焦点的位置。
反射镜450起调整光路的作用，激光器410为可调谐激光器，以产生适合聚焦涡旋光发生器100设计波长的激光，物镜440为40×倍镜头，图像传感器470为红外CCD(Charge Coupled Device，电荷耦合器件)传感器。
请参照图10，为通过上述干涉检测系统判断实际聚焦点的成像图，其中设计波长λ为1550nm，设计焦距f为20μm，离散数M为2。由图10可知，对于拓扑荷数p＝0的涡旋光，于16.6μm处聚焦，焦距为16.6μm；对于拓扑荷数p＝1的涡旋光，于15.8μm处聚焦，焦距为15.8μm；对于拓扑荷数p＝2的涡旋光，于15.4μm处聚焦，焦距为15.4μm。由于复合型螺旋相位片150微结构亚波长特征尺寸内存在倏逝场的衍射，所以实际焦距与设计焦距20μm存在一定偏差，通过仿真计算可以预测偏差的大小。
拓扑荷数p为1的涡旋光的干涉图像如图10所示，图10中的干涉条纹出现一处脱臼，如图10中P1位置所示。拓扑荷数p为2的涡旋光的干涉图像如图11所示，图11中有干涉条纹出现两处脱臼，如图11中P2位置所示。可以理解的是，干涉图像中脱臼处的数量相等于涡旋光的拓扑荷数p。依此干涉图样可以检测并判断出聚焦涡旋光发生器100产生聚焦涡旋光的拓扑荷数。
以上所述实施例的各技术特征可以进行任意的组合，为使描述简洁，未对上述实施例中的各个技术特征所有可能的组合都进行描述，然而，只要这些技术特征的组合不存在矛盾，都应当认为是本说明书记载的范围。
以上所述实施例仅表达了本发明的几种实施方式，其描述较为具体和详细，但并不能因此而理解为对发明专利范围的限制。应当指出的是，对于本领域的普通技术人员来说，在不脱离本发明构思的前提下，还可以做出若干变形和改进，这些都属于本发明的保护范围。因此，本发明专利的保护范围应以所附权利要求为准。

Claims (13)

  1. 一种聚焦涡旋光发生器，包括：单模光纤、多模光纤和复合型螺旋相位片，所述多模光纤的输入端与所述单模光纤对接耦合，所述多模光纤的输出端与所述复合型螺旋相位片对接耦合。
  2. 根据权利要求1所述的聚焦涡旋光发生器，其特征在于，所述多模光纤为渐变折射率光纤，所述渐变折射率光纤的长度为节距的(2N+1)/4倍，其中N为自然数。
  3. 根据权利要求1所述的聚焦涡旋光发生器，其特征在于，所述复合型螺旋相位片的透过率函数
    Figure PCTCN2020077903-appb-100001
    满足如下关系：
    Figure PCTCN2020077903-appb-100002
    其中p为拓扑荷数，λ为被调制激光的波长，f为所述复合型螺旋相位片的设计焦距，
    Figure PCTCN2020077903-appb-100003
    为极坐标，r为极径，
    Figure PCTCN2020077903-appb-100004
    为极角，i为虚数单位，D为大于零的常数，circ(r/R)为圆域函数，R为复合型螺旋相位片的半径。
  4. 根据权利要求1所述的聚焦涡旋光发生器，其特征在于，所述复合型螺旋相位片的极坐标上厚度分布h满足如下关系：
    Figure PCTCN2020077903-appb-100005
    其中，
    Figure PCTCN2020077903-appb-100006
    其中，λ为被调制激光的波长，n 1为固化后所述光敏胶的折射率，n 2为所述复合型螺旋相位片周围介质的折射率，p为拓扑荷数，f为所述复合型螺旋相位片的设计焦距，
    Figure PCTCN2020077903-appb-100007
    为极坐标，r为极径，
    Figure PCTCN2020077903-appb-100008
    为极角，i为虚数单位，D为大于零的常数。
  5. 根据权利要求4所述的聚焦涡旋光发生器，其特征在于，所述复合型螺旋相位片分布有槽体结构，所述槽体结构的槽深h 0满足如下关系：
    Figure PCTCN2020077903-appb-100009
    其中，λ为被调制激光的波长，n 1为所述复合型螺旋相位片的折射率，n 2为所述复合型螺旋相位片周围介质的折射率。
  6. 根据权利要求1所述的聚焦涡旋光发生器，其特征在于，所述复合型螺旋相位片的直径大于所述多模光纤的有效通光直径，且小于所述多模光纤的外径。
  7. 根据权利要求1所述的聚焦涡旋光发生器，其特征在于，所述聚焦涡旋光发生器还包括匹配层，所述匹配层设置于所述多模光纤与所述复合型螺旋相位片之间，使所述复合型螺旋相位片牢固的粘附在所述多模光纤上。
  8. 一种聚焦涡旋光发生器的制备方法，包括:
    提供单模光纤和多模光纤；
    获取复合型螺旋相位片的结构数据；
    熔接所述单模光纤的输出端和所述多模光纤的输入端；
    依据所述复合型螺旋相位片的结构数据在所述多模光纤的输出端制备所述复合型螺旋相位片。
  9. 根据如权利要求8所述的制备方法，其特征在于，制备所述复合型螺旋相位片的方法包括：提供光敏胶；固定所述多模光纤，并使所述多模光纤的输出端接触所述光敏胶；使用激光固化所述光敏胶，固化后的所述光敏胶形成所述复合型螺旋相位片的结构；去除残余的光敏胶。
  10. 根据如权利要求9所述的制备方法，其特征在于，所述去除残余的 光敏胶的方法包括：将所述多模光纤的输出端浸入丙二醇甲醚乙酸酯中以除去未曝光的所述光敏胶；将经丙二醇甲醚乙酸酯处理过的所述多模光纤的输出端放入异丙醇溶液中处理，以洗去丙二醇甲醚乙酸酯；用氮气吹干所述多模光纤的输出端。
  11. 根据如权利要求8所述的制备方法，其特征在于，所述获取复合型螺旋相位片的结构数据的方法包括：获取所述聚焦涡旋光发生器所应用的光波波长，所需要的焦距，以及所述多模光纤的有效通光直径；依据所述波长和焦距计算得到所述复合型螺旋相位片的相位函数；依据所述相位函数得到所述复合型螺旋相位片的极坐标上厚度分布数据，以及依据所述有效通光直径得到所述复合型螺旋相位片的边界直径。
  12. 根据如权利要求11所述的制备方法，其特征在于，所述获取复合型螺旋相位片的结构数据的方法还包括：对所述复合型螺旋相位片的极坐标上连续的厚度分布数据进行离散化处理。
  13. 根据如权利要求11所述的制备方法，其特征在于，所述复合型螺旋相位片的极坐标上厚度分布h满足如下关系：
    Figure PCTCN2020077903-appb-100010
    其中，
    Figure PCTCN2020077903-appb-100011
    其中，λ为被调制激光的波长，n 1为固化后所述光敏胶的折射率，n 2为所述复合型螺旋相位片周围介质的折射率，p为拓扑荷数，f为所述复合型螺旋相位片的设计焦距，
    Figure PCTCN2020077903-appb-100012
    为极坐标，r为极径，
    Figure PCTCN2020077903-appb-100013
    为极角，i为虚数单位，其中D为大于零的常数。
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