All-fiber optical wrench and method of using the same

Abstract

The present invention discloses an optical wrench, comprising a combined optical fiber; a spiral long-period fiber grating, used for mode coupling a resonant wavelength light beam transmitted in the combined optical fiber to form a first vortex light beam; a spiral zone plate, used for topological charge conversion of the first vortex light beam and focusing to form a first focused vortex light beam, and spiral phase modulation of a non-resonant wavelength light beam emitted from the combined optical fiber and focusing to form a second focused vortex light beam. The all-fiber optical wrench can output a first focused vortex light beam and a second focused vortex light beam with opposite topological charges, and then use the first focused vortex light beam and the second focused vortex light beam to drive the particles to achieve forward rotation, reverse rotation and stop rotation. The present invention also provides and a method of using the same, which are applied to the above-mentioned all-fiber optical wrench.

Description

Technical Field

[0001] The invention relates to an optical wrench, and in particular to an all-fiber optical wrench and a use method thereof. Background Art

[0002] Optical tweezers can use the gradient force formed by the spatial variation of the light field intensity to stably capture particles at the strongest point of the light field, that is, the focal position of the light beam. When the laser beam moves, it can drive the particles to move together, realizing three-dimensional precision manipulation of the particles. Since the invention of optical tweezers technology by Bell Labs scientist A. Ashkin in 1986, its research and application have developed rapidly, especially in the field of life sciences. Optical tweezers have become an indispensable tool that can be used to manipulate particles, study the static and dynamic mechanical behavior of biological particles, and even perform fine manipulation of biological macromolecules.

[0003] The optical wrench is based on the optical tweezers technology. It uses a light beam with angular momentum (AM) to illuminate the particle. While trapping the particle, it transfers part of the angular momentum to the particle, causing it to rotate. In other words, it adds one-dimensional angular manipulation to the three-dimensional manipulation of particles by optical tweezers. In 1995, He et al. first observed the transfer of photon orbital angular momentum to particles in an experiment, and realized the manipulation of tiny particles using optical helices.

[0004] A Chinese patent with patent number 202210081055.4 discloses a single-fiber vortex optical tweezers, including a single-mode optical fiber, a gradient-index multimode optical fiber and a spiral zone plate, wherein one end face of the single-mode optical fiber is fused with one end face of the gradient-index multimode optical fiber, and the spiral zone plate is prepared on the other end face of the gradient-index multimode optical fiber; the spiral zone plate can perform spiral phase modulation and diffraction focusing on the light beam emitted from the gradient-index multimode optical fiber, so that the light beam forms a focused vortex beam, and then use the formed focused vortex beam to drive the particles to perform three-dimensional motion and rotation.

[0005] However, the single-fiber vortex optical tweezers of the above-mentioned patent can only achieve single-wavelength input, that is, the single-fiber vortex optical tweezers can only output a focused vortex light beam rotating in a single direction, such as forward or reverse rotation. Specifically, it can only drive the particles to rotate in one direction, but cannot drive the particles to rotate in another direction, nor can it drive the rotating particles to stop rotating. Summary of the invention

[0006] In order to solve the deficiencies of the above-mentioned prior art, the present invention provides an all-fiber optical wrench, which can output a first focused vortex beam and a second focused vortex beam with opposite topological charges, and then use the first focused vortex beam and the second focused vortex beam to drive the particles to achieve forward rotation, reverse rotation and stop rotation.

[0007] The invention also provides a method for using the all-fiber optical wrench.

[0008] The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0009] An optical wrench, comprising:

[0010] A combined optical fiber having an incident end face and an output end face for transmitting a light beam;

[0011] A spiral long-period fiber grating is arranged between the incident end face and the output end of the combined optical fiber, and is used for mode coupling the resonant wavelength light beam transmitted in the combined optical fiber to form a first vortex light beam;

[0012] The spiral zone plate is arranged on the output end face of the combined optical fiber, and is used to perform topological charge conversion on the first vortex light beam and focus it to form a first focused vortex light beam, and to perform spiral phase modulation on the non-resonant wavelength light beam output from the combined optical fiber and focus it to form a second focused vortex light beam.

[0013] The method for using the all-fiber optical wrench comprises the following steps:

[0014] coupling the resonant wavelength light beam and the non-resonant wavelength light beam into the combined optical fiber in a predetermined ratio;

[0015] The spiral long-period fiber grating is used to perform mode coupling on the resonant wavelength light beam transmitted in the combined optical fiber to form a first vortex light beam;

[0016] The spiral zone plate is used to perform topological charge conversion on the first vortex beam emitted from the combined optical fiber and focus it to form a first focused vortex beam, and the non-resonant wavelength beam emitted from the combined optical fiber is spirally phase modulated and focused to form a second focused vortex beam, and the first focused vortex beam and the second focused vortex beam are applied to the particle to drive the particle to rotate.

[0017] The present invention has the following beneficial effects: the all-fiber optical wrench of the present invention uses the spiral long-period fiber grating and the spiral zone plate in combination, the spiral long-period fiber grating can excite the fundamental mode in the optical fiber into a high-order mode with orbital angular momentum, so as to perform mode coupling on the resonant wavelength light beam to convert it into a first vortex light beam with a high-order mode, and the spiral zone plate has a spiral phase, which can add a spiral phase with orbital angular momentum to all light beams, so as to perform topological charge conversion on the first vortex light beam and perform spiral phase modulation on the non-resonant wavelength light beam and focus to form the first focused spiral light beam and the second focused spiral light beam respectively, and through the cooperation of the spiral long-period fiber grating and the spiral zone plate, the first focused vortex light beam and the second focused vortex light beam with opposite topological charges can be output, and then the first focused vortex light beam and the second focused vortex light beam are used to drive the particles to realize forward rotation, reverse rotation and stop rotation. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic structural diagram of the all-fiber optical wrench provided by the present invention.

[0019] Figure 2 This is a schematic diagram of the structure of the spiral zone plates in the all-fiber optical wrench provided by the present invention when they are 0th order, 1st order and 2nd order respectively.

[0020] Figure 3 This is a schematic diagram of the principle of the CO2 laser system provided by the present invention.

[0021] Figure 4 A schematic diagram of the steps of using the all-fiber optical wrench provided by the present invention. DETAILED DESCRIPTION

[0022] The present invention is described in detail below in conjunction with the accompanying drawings and embodiments, examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0023] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside" and "outside" etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0024] In addition, the terms "first", "second", and "third" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second", and "third" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0025] In the present invention, unless otherwise clearly specified and limited, the terms "install", "connect", "connect", "fix", "set" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0026] Embodiment 1

[0027] like Figure 1 As shown, an optical wrench comprises:

[0028] The combined optical fiber 100 has an incident end face and an output end face, and is used to transmit a light beam;

[0029] The spiral long-period fiber grating 200 is disposed between the incident end face and the output end of the combined optical fiber 100 and is used for mode coupling the resonant wavelength light beam λ1 transmitted in the combined optical fiber 100 to form a first vortex light beam;

[0030] The spiral zone plate 300 is arranged on the output end face of the combined optical fiber 100, and is used to perform topological charge conversion on the first vortex beam and focus it to form a first focused vortex beam, and to perform spiral phase modulation on the non-resonant wavelength beam λ2 output from the combined optical fiber 100 and focus it to form a second focused vortex beam.

[0031] The all-fiber optical wrench of the present invention uses the spiral long-period fiber grating 200 and the spiral zone plate 300 in combination. The spiral long-period fiber grating 200 can excite the fundamental mode in the optical fiber into a high-order mode with orbital angular momentum, so as to perform mode coupling on the resonant wavelength light beam λ1 to convert it into a first vortex light beam with a high-order mode. The spiral zone plate 300 has a spiral phase and can add a spiral phase with orbital angular momentum to all light beams to perform topological charge conversion on the first vortex light beam and perform spiral phase modulation on the non-resonant wavelength light beam λ2 and focus to form the first focused spiral light beam and the second focused spiral light beam respectively. Through the cooperation of the spiral long-period fiber grating 200 and the spiral zone plate 300, the first focused vortex light beam and the second focused vortex light beam with opposite topological charges can be output, and then the first focused vortex light beam and the second focused vortex light beam can be used to drive the particles to achieve forward rotation, reverse rotation and stop rotation.

[0032] The spiral zone plate 300 has a dual function: on the one hand, for the first vortex light beam transmitted by the combined optical fiber 100, the spiral zone plate 300 can realize topological charge conversion and further focus it to generate the first focused vortex light beam; on the other hand, for the non-resonant wavelength light beam λ2 transmitted by the combined optical fiber 100, the spiral zone plate 300 is responsible for spiral phase modulation thereof and then focusing it as well to construct the second focused vortex light beam.

[0033] The spiral long-period fiber grating 200 has a resonant wavelength and a non-resonant wavelength. When the resonant wavelength beam λ1 is input into the combined optical fiber 100, the spiral long-period fiber grating 200 can generate a high-order mode (i.e., a first vortex beam) with orbital angular momentum, while when the non-resonant wavelength beam λ2 is input into the combined optical fiber 100, the spiral long-period fiber grating 200 does not undergo mode coupling and only outputs a Gaussian beam of the fundamental mode. That is, the spiral long-period fiber grating 200 can only perform mode coupling on the resonant wavelength beam λ1, but cannot perform mode coupling on the non-resonant wavelength beam λ2.

[0034] When in use, if the resonant wavelength light beam λ1 is coupled into the combined optical fiber 100, when the resonant wavelength light beam λ1 passes through the spiral long-period fiber grating 200, since its wavelength is the same as the resonant wavelength of the spiral long-period fiber grating 200, it can carry orbital angular momentum through the mode coupling of the spiral long-period fiber grating 200, and then be converted into the first vortex light beam. When the first vortex light beam is emitted from the exit end face of the combined optical fiber 100, it is modulated by the spiral phase of the spiral zone plate 300 and the order of the spiral zone plate 300 is superimposed to realize the conversion of the topological charge number. Finally, the first vortex light beam is transmitted through the spiral phase modulation of the spiral zone plate 300. The first focused vortex beam is formed by the diffraction focusing of the spiral zone plate 300; if the non-resonant wavelength beam λ2 is coupled into the combined optical fiber 100, when the non-resonant wavelength beam λ2 passes through the spiral long-period fiber grating 200, since its wavelength is different from the resonant wavelength of the spiral long-period fiber grating 200, it cannot be mode-coupled through the spiral long-period fiber grating 200, so it still remains a Gaussian beam of the base film. When it is emitted from the output end face of the combined optical fiber 100, it is spirally phase modulated and diffraction-focused by the spiral zone plate 300 to form the second focused vortex beam.

[0035] The motion state of the particle is determined by the combined action of the first focused vortex light beam λ1 and the second focused vortex light beam λ2.

[0036] In one of the methods of use of the present invention, the resonant wavelength light beam λ1 and the non-resonant wavelength light beam λ2 are coupled together into the combined optical fiber 100, so that the all-fiber optical wrench simultaneously outputs a symmetrical first focused vortex light beam and a second focused vortex light beam, and by adjusting the output power of the resonant wavelength light beam λ1 and the non-resonant wavelength light beam λ2, the magnitude of the torque formed by the first focused vortex light beam and the second focused vortex light beam on the particle is controlled.

[0037] For example, the order of the spiral long-period fiber grating 200 is +2, and the order of the spiral zone plate 300 is -1. After the mode coupling of the spiral long-period fiber grating 200, the resonant wavelength light beam λ1 forms a first vortex light beam with a topological charge of +2. After the spiral phase modulation and diffraction focusing of the spiral zone plate 300, the first vortex light beam is converted into a first focused vortex light beam with a topological charge of +1; and the non-resonant wavelength light beam λ2 forms a second focused vortex light beam with a topological charge of -1 after the spiral phase modulation and focusing diffraction of the spiral zone plate 300. It can be seen that the first focused vortex light beam and the second focused vortex light beam have opposite order directions but the same order values, and a symmetrical relationship between the positive rotation and the negative rotation.

[0038] When the particle is in a stationary state, if the output power of the resonant wavelength beam λ1 is greater than the output power of the non-resonant wavelength beam λ2, the force of the first focused vortex beam is greater than the force of the second focused vortex beam, and the particle obtains a forward angular momentum; if the output power of the resonant wavelength beam λ1 is less than the output power of the non-resonant wavelength beam λ2, the force of the first focused vortex beam is less than the force of the second focused vortex beam, and the particle obtains a reverse angular momentum; when the particle is in a forward rotating state, if the output power of the resonant wavelength beam λ1 is less than the output power of the non-resonant wavelength beam λ2, the force of the first focused vortex beam is less than the force of the second focused vortex beam, and the particle obtains a reverse angular momentum, and its rotation speed gradually slows down under the reverse angular momentum, and eventually stops rotating; when the particle is in a reverse rotating state, if the output power of the resonant wavelength beam λ1 is greater than the output power of the non-resonant wavelength beam λ2, the force of the first focused vortex beam is greater than the force of the second focused vortex beam, and the particle obtains a forward angular momentum, and its rotation speed gradually slows down under the forward angular momentum, and eventually stops rotating.

[0039] In another method of using the present invention, by controlling the on and off of the resonant wavelength beam λ1 and the non-resonant wavelength beam λ2, one of the resonant wavelength beam λ1 and the non-resonant wavelength beam λ2 is selectively coupled into the combined optical fiber 100, and the all-fiber optical wrench selectively outputs one of the symmetrical first focused vortex beam and the second focused vortex beam.

[0040] For example, when the order of the spiral long-period fiber grating 200 is +2 and the order of the spiral zone plate 300 is -1, the resonant wavelength light beam λ1 forms a first vortex light beam with a topological charge of +2 after mode coupling of the spiral long-period fiber grating 200, and the first vortex light beam is converted into a first focused vortex light beam with a topological charge of +1 after spiral phase modulation and diffraction focusing of the spiral zone plate 300; and the non-resonant wavelength light beam λ2 forms a second focused vortex light beam with a topological charge of -1 after spiral phase modulation and focusing diffraction of the spiral zone plate 300. It can be seen that the first focused vortex light beam and the second focused vortex light beam have opposite order directions and the same order values, and there is a symmetrical relationship between the positive rotation and the negative rotation.

[0041] When the particle is in a stationary state, if the non-resonant wavelength light beam λ2 is controlled to be disconnected and only the resonant wavelength light beam λ1 is coupled into the combined optical fiber 100, the all-fiber optical wrench outputs a first focused vortex light beam with a topological charge of +1 to drive the particle to rotate forwardly. If the resonant wavelength light beam λ1 is controlled to be disconnected and only the non-resonant wavelength light beam λ2 is coupled into the combined optical fiber 100, the all-fiber optical wrench outputs a second focused vortex light beam with a topological charge of -1 to drive the particle to rotate reversely. When the particle is in a forward rotating state, if the resonant wavelength light beam λ1 is controlled to be disconnected and only the non-resonant wavelength light beam λ2 is coupled into the combined optical fiber 100, the all-fiber optical wrench outputs a second focused vortex light beam with a topological charge of -1 to drive the particle to rotate reversely. If the non-resonant wavelength light beam λ2 is coupled into the combined optical fiber 100, the all-fiber optical wrench alone outputs a second focused vortex light beam with a topological charge of -1, the particle obtains a reverse angular momentum, and its rotation speed gradually slows down under the reverse angular momentum, and finally stops rotating; when the particle is in a reverse rotation state, if the non-resonant wavelength light beam λ2 is controlled to be disconnected and only the resonant wavelength light beam λ1 is coupled into the combined optical fiber 100, the all-fiber optical wrench alone outputs a first focused vortex light beam with a topological charge of +1, the particle obtains a forward angular momentum, and its rotation speed gradually slows down under the forward angular momentum, and finally stops rotating.

[0042] Of course, the angular momentum of the vortex beam is also related to the value of its topological charge. The larger the value of the topological charge, the larger the angular momentum of the vortex beam. Therefore, in another method of use of the present invention, the order of the spiral long-period fiber grating 200 and the spiral wave zone plate 300 can also be matched according to the angular momentum required by the first focused vortex beam and the second focused vortex beam, so that the all-fiber optical wrench can simultaneously output an asymmetric first focused vortex beam and a second focused vortex beam, or selectively output one of the asymmetric first focused vortex beam and the second focused vortex beam to meet specific application scenarios.

[0043] For example, the order of the spiral long-period fiber grating 200 is -3, and the order of the spiral zone plate 300 is +1. After the mode coupling of the spiral long-period fiber grating 200, the resonant wavelength light beam λ1 forms a first vortex light beam with a topological charge of -3. After the spiral phase modulation and focusing diffraction of the spiral zone plate 300, the first vortex light beam is converted into a first focused vortex light beam with a topological charge of -2; and the non-resonant wavelength light beam λ2 forms a second focused vortex light beam with a topological charge of +1 after the spiral phase modulation and focusing diffraction of the spiral zone plate 300. It can be seen that the order directions of the first focused vortex light beam and the second focused vortex light beam are opposite and the order values ​​are different, and the positive rotation and the negative rotation are in an asymmetric relationship.

[0044] In summary, the spiral long-period fiber grating 200 and the spiral zone plate 300 can be combined in a variety of orders according to needs, but the order matching between the spiral long-period fiber grating 200 and the spiral zone plate 300 requires that the order directions of the first focused vortex beam and the second focused vortex beam are opposite. Because, if the order directions of the spiral long-period fiber grating 200 and the spiral zone plate 300 are the same, for example, the order of the spiral long-period fiber grating 200 is +1, and the order of the spiral zone plate 300 is also +1, then the topological sum of the first focused vortex beam and the second focused vortex beam output by the all-fiber optical wrench are +2 and +1 respectively, then it can only drive the stationary particles to rotate forward or drive the reverse rotating particles to stop rotating, but it cannot drive the stationary particles to rotate reversely or drive the forward rotating particles to stop rotating; and if the If the order values ​​of the spiral long-period fiber grating 200 and the spiral zone plate 300 are the same, for example, the order of the spiral long-period fiber grating 200 is +1, and the order of the spiral zone plate 300 is -1, then the orders of the first focused vortex beam and the second focused vortex beam output by the all-fiber optical wrench are 0 and -1, that is, the first focused vortex beam cannot be output, and it can only drive the stationary particles to rotate reversely or drive the forward rotating particles to stop rotating, but cannot drive the stationary particles to rotate forwardly or drive the reverse rotating particles to stop rotating.

[0045] The combined optical fiber 100 includes a first optical fiber 110 and a graded-index multimode optical fiber 120, wherein the other end face of the first optical fiber 110 opposite to its connection end face serves as the incident end face, and the other end face of the graded-index multimode optical fiber 120 opposite to its connection end face serves as the output end face, and the spiral long-period fiber grating 200 is formed in the core or cladding of the first optical fiber.

[0046] The refractive index of the graded-index multimode optical fiber 120 is the highest at the center and is gradually decreasing along the radial direction. The propagation path of the light beam in the graded-index multimode optical fiber 120 presents a sinusoidal waveform. After the light beam is coupled from the first optical fiber 110 into the graded-index multimode optical fiber 120, due to the gradual change of the refractive index of the graded-index multimode optical fiber 120, its light beam mode field is expanded, achieving a beam expansion effect, so that the spiral zone plate 300 has a larger design space. This is conducive to forming a sufficiently large modulation range on the output end face to design a spiral zone plate 300 with a high numerical aperture, so as to integrate more wavebands to modulate the light beam, so as to generate a strongly focused vortex beam to perform rotational manipulation of the particles.

[0047] In this embodiment, the other end face of the first optical fiber 110 opposite to the incident end face is directly fused with the other end face of the graded-index multimode optical fiber 120 opposite to the emitting end face.

[0048] During preparation, the first optical fiber 110 and the graded-index multimode optical fiber 120 may be fusion-spliced ​​by a fiber fusion splicer, and then the graded-index multimode optical fiber 120 may be cut to a desired length by a fiber precision cutter.

[0049] The first optical fiber 110 includes a first core 111 and a first cladding 112, and the first cladding 112 surrounds the outer circumference of the first core 111; the graded-index multimode optical fiber 120 includes a second core 121 and a second cladding 122, and the second cladding 122 surrounds the outer circumference of the second core 121; the diameter of the second core 121 is greater than the diameter of the first core 111, and the outer diameter of the first cladding 112 is equal to the outer diameter of the second cladding 122.

[0050] Because the core diameter of the first optical fiber 110 is smaller than the core diameter of the graded-index multimode optical fiber 120, during preparation, it is only necessary to consider aligning the outer diameter of the first optical fiber 110 with the outer diameter of the graded-index multimode optical fiber 120, so that the first optical fiber 110 and the graded-index multimode optical fiber 120 can be directly aligned and fused by a fusion splicer. This method is simple, easy to operate and low in cost. At the connection portion after direct fusion splicing, the second core 121 and the first core 111 are in a stepped shape.

[0051] Among them, the length of the graded-index multimode optical fiber 120 is equal to or less than (2n+1) / 4 pitches, where the pitch is the unit optical fiber length corresponding to a complete sinusoidal wave cycle transmitted by the light beam in the graded-index multimode optical fiber 120, and n is a natural number.

[0052] Optimally, when the length of the graded-index multimode optical fiber 120 is equal to (2n+1) / 4 pitches, the light beam emitted from the graded-index multimode optical fiber 120 is exactly at the peak of the sine wave. At this time, the lateral range of the light beam transmitted in the graded-index multimode optical fiber is the largest, and the beam expansion effect is the best. At the same time, the slope of the sine wave peak of the light beam at the exit end face of the graded-index multimode optical fiber 120 is 0, so during the light beam transmission process, it can be understood that the divergence angle of the light beam is close to 0, which means that the light beam is similar to parallel light when it is emitted, and the collimation effect is also the best.

[0053] During preparation, when determining the length of (2n+1) / 4 pitches of the graded-index multimode optical fiber 120, the following steps are included:

[0054] Prepare a plurality of combined optical fiber 100 samples, wherein the lengths of the graded-index optical fibers in different combined optical fiber 100 samples are different, wherein the lengths of the graded-index multimode optical fibers 120 in different combined optical fiber 100 samples start with L1, and are sampled at preset intervals, and the lengths are L1, 2*L1, 3*L1, ..., 20*L1 in sequence; the range of L1 is 10-100 μm, and L1 is further preferably 30-70 μm;

[0055] The divergence angles of the output end faces of the graded-index multimode optical fibers 120 in different combined optical fiber 100 samples are measured, and the length of the graded-index multimode optical fiber 120 in the combined optical fiber 100 sample with the smallest divergence angle is determined to be (2n+1) / 4 pitches.

[0056] Specifically, the method for measuring the divergence angle includes the following steps:

[0057] The graded-index multimode optical fiber 120 in the combined optical fiber 100 sample is fixed on a displacement platform, with the output end face facing the CCD camera, and then the incident end face of the first optical fiber 110 in the combined optical fiber 100 sample is connected to a single-wavelength light source, the wavelength of which is the corresponding design wavelength of the spiral zone plate 300;

[0058] Starting from the distance L2, the distance between the output end face of the gradient refractive index multimode optical fiber 120 and the CCD camera is sequentially increased by the displacement platform and the spot image is recorded each time, wherein the range of L2 is 1 to 10 mm;

[0059] Through software processing, the change rate of the spot diameter as the distance between the exit end face and the CCD camera changes is obtained. Half of this change rate is the tangent value of the half divergence angle of the gradient refractive index multimode optical fiber 120. The size of the divergence angle can be obtained based on the tangent value.

[0060] Specifically, according to the formula Get the size of the divergence angle, where θ is the beam divergence angle, is the rate of change of the spot diameter as the distance from the camera changes. Since the divergence angle is very small, the tangent value of the divergence angle is approximately equal to the size of the divergence angle, and the divergence angle is approximately:

[0061] The first optical fiber 110 is a single-mode optical fiber, a few-mode optical fiber, a combination of a single-mode optical fiber and a few-mode optical fiber, or a combination of a single-mode optical fiber and a ring-core optical fiber. The core of the single-mode optical fiber only has the fundamental mode. Although there are other high-order modes in the core of the few-mode optical fiber, the fundamental mode is still the main mode, and the proportion of other modes is very small. The ring-core optical fiber is a special type of few-mode optical fiber. Therefore, writing the spiral long-period fiber grating 200 on the first optical fiber 110 can excite the fundamental mode in the first optical fiber 110 into a high-order mode carrying orbital angular momentum. The spiral long-period fiber grating 200 satisfies the following formula:

[0062] N CO -N F =m·λ / Λ0

[0063] J CO =J F +m·σ

[0064] Where N CO 、N F is the effective refractive index of the fundamental mode and the effective refractive index of the higher-order mode in the first optical fiber 110, m is the harmonic order of the spiral long-period fiber grating 200, λ is the resonant wavelength of the spiral long-period fiber grating 200, Λ0 is the period of the spiral long-period fiber grating, J CO and J F is the total angular momentum of the fundamental mode and the higher-order modes in the first optical fiber 110 (the sum of the spin angular momentum and the orbital angular momentum), σ=±1 is the spiral direction of the spiral long-period fiber grating 200, + is the positive rotation (also called positive rotation, left rotation or clockwise rotation), and - is the reverse rotation (also called reverse rotation, right rotation or counterclockwise rotation).

[0065] In this embodiment, the spiral long-period fiber grating 200 is prepared using a CO2 laser system. Figure 3As shown, the CO2 laser system includes a CO2 laser 1, a reflector 2, an aperture 3, a shutter 4, a beam expander 5, a focusing lens 6, a rotating fixture 7, a displacement platform 8, a broadband light source 9, a spectrometer 10 and a computer 11; the CO2 laser 1, the reflector 2, the aperture 3, the shutter 4, the beam expander 5 and the focusing lens 6 are arranged in sequence along the transmission path of the CO2 laser, the rotating fixture 7 is arranged on the displacement platform 8, the broadband light source 9 is connected to one end face of the first optical fiber 110, the spectrometer 10 is connected to the other end face of the first optical fiber 110, and the computer 11 is connected to control the CO2 laser 1, the rotating fixture 7 and the displacement platform 8; the rotating fixture 7 is used to clamp the first optical fiber 110 and drive the first optical fiber 11 0 rotates around its axis, the displacement platform 8 is used to drive the first optical fiber 110 on the rotating fixture 7 to move along its axis; the CO2 laser emitted by the CO2 laser 1 sequentially passes through the reflector 2, the aperture 3, the shutter 4, the beam expander 5 and the focusing lens 6 and then focuses on the first optical fiber 110; in the process of preparing the spiral long-period fiber grating 200, the computer 11 controls the moving speed of the displacement platform 8 and the rotating speed of the rotating fixture 7, the broadband light source 9 reflects the broadband light beam into the first optical fiber 110, and the spectrometer 10 obtains the transmission spectrum of the first optical fiber 110 in the process of writing the spiral grating, and the writing condition of the spiral long-period fiber grating 200 can be determined according to the spectrum of the transmission spectrum.

[0066] The spiral zone plate 300 includes two parts: a spiral phase structure and a Fresnel zone structure. The spiral phase structure is used to add a spiral phase to the light beam, and its transmittance function is exp(ilφ). The Fresnel zone structure is used to diffract and focus the light beam, and its transmittance function is Therefore, the transmittance function of the spiral zone plate 300 satisfies the following formula:

[0067]

[0068] Wherein λ and f are the wavelength and focal length of the focused vortex beam (the first focused vortex beam or the second focused vortex beam), l is the waveband order of the spiral zone plate 300, and r and φ are polar coordinates. Figure 2 The spiral zone plate 300 is shown with the wave zone order l being 0, 1 and 2. It can be seen that when the wave zone order l=0, the structure of the spiral zone plate 300 is a Fresnel zone plate with a concentric circle structure.

[0069] In this embodiment, the spiral zone plate 300 is prepared by focused ion beam etching technology (FIB). First, a light-shielding film layer is formed on the output end face of the gradient refractive index multimode optical fiber 120. The light-shielding film layer is preferably a metal film layer. The metal film layer is firmly combined with the optical fiber end face and can block light with stable performance. For example, the metal film layer can be a gold film, a silver film or a copper film, etc., with a thickness between 50nm-150nm; then the high-energy ion beam is focused on the light-shielding film layer, and the gradient refractive index multimode optical fiber 120 is driven to move by a three-dimensional displacement platform, so that the high-energy ion beam is The shape of the spiral wave zone plate 300 is etched on the shading film layer, and the spiral wave zone plate 300 includes a light-transmitting wave zone and a light-shielding wave zone. The light-transmitting wave zone is formed by the area on the light-shielding film layer that is removed by the high-energy ion beam etching, and the light-shielding wave zone is formed by the area on the light-shielding film layer that is not etched by the high-energy ion beam and retained. The light-transmitting wave zone and the light-shielding wave zone are arranged in sequence at intervals along the circumference of the end face of the gradient refractive index multimode optical fiber 120 and are spirally wound around each other to form an amplitude-type wave zone plate.

[0070] Embodiment 2

[0071] The method for using the all-fiber optical wrench described in Example 1 is as follows: Figure 4 As shown, the method of use includes the following steps:

[0072] Step 100: coupling the resonant wavelength light beam λ1 and the non-resonant wavelength light beam λ2 into the combined optical fiber 100 according to a predetermined ratio;

[0073] Step 200: using the spiral long-period fiber grating 200 to perform mode coupling on the resonant wavelength light beam λ1 transmitted in the combined optical fiber 100 to form a first vortex light beam;

[0074] Step 300: The spiral zone plate 300 is used to perform topological charge conversion on the first vortex beam emitted from the combined optical fiber 100 and focus it to form a first focused vortex beam, and the non-resonant wavelength beam λ2 emitted from the combined optical fiber 100 is spirally phase modulated and focused to form a second focused vortex beam, and the first focused vortex beam and the second focused vortex beam are applied to the particle to drive the particle to rotate.

[0075] In this embodiment, the resonant wavelength light beam λ1 and the non-resonant wavelength light beam λ2 are coupled into the combined optical fiber 100 in a ratio of 50:50.

[0076] In step 300, when the particle is driven to rotate, the torque generated by the first focused vortex beam and the second focused vortex beam on the rotating platform is controlled by adjusting the output power of the resonant wavelength beam λ1 and the non-resonant wavelength beam λ2, or one of the first focused vortex beam and the second focused vortex beam is selectively output by controlling the on-off state of the resonant wavelength beam λ1 and the non-resonant wavelength beam λ2.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention rather than to limit them. Although the embodiments of the present invention have been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical wrench, characterized in that: include: A combined optical fiber having an incident end face and an output end face for transmitting a light beam; A spiral long-period fiber grating is arranged between the incident end face and the output end of the combined optical fiber, and is used for mode coupling the resonant wavelength light beam transmitted in the combined optical fiber to form a first vortex light beam; A spiral zone plate, arranged on the output end face of the combined optical fiber, for performing topological charge conversion on the first vortex light beam and focusing it to form a first focused vortex light beam, and performing spiral phase modulation on the non-resonant wavelength light beam output from the combined optical fiber and focusing it to form a second focused vortex light beam; The orders of the first focused vortex beam and the second focused vortex beam are opposite in direction.

2. The optical wrench according to claim 1, characterized in that: The combined optical fiber includes a first optical fiber and a graded-index multimode optical fiber, and the first optical fiber and the graded-index multimode optical fiber are end-faced connected; the other end face of the first optical fiber opposite to its connected end face serves as the incident end face, and the other end face of the graded-index multimode optical fiber opposite to its connected end face serves as the exit end face, and the helical long-period fiber grating is formed in the core or cladding of the first optical fiber.

3. The optical wrench according to claim 2, characterized in that: The first optical fiber is a single-mode optical fiber, a few-mode optical fiber, a combination of a single-mode optical fiber and a few-mode optical fiber, or a combination of a single-mode optical fiber and a ring-core optical fiber.

4. The optical wrench according to claim 2 or 3, characterized in that: The spiral long-period fiber grating satisfies the following formula: N CO -N F =m·λ / Λ0 I CO =J F +m·σ Where N CO 、N F is the effective refractive index of the fundamental mode and the effective refractive index of the higher-order mode in the first optical fiber, m is the topological order of the spiral long-period fiber grating, λ is the resonant wavelength of the spiral long-period fiber grating, Λ0 is the period of the spiral long-period fiber grating, J CO and J F is the total angular momentum of the fundamental mode and the higher-order modes in the first optical fiber, and σ=±1 is the helical direction of the helical long-period fiber grating.

5. The optical wrench according to claim 2, characterized in that: The length of the graded-index multimode optical fiber is equal to (2n+1) / 4 pitches, where the pitch is the unit optical fiber length corresponding to a complete sinusoidal wave cycle transmitted by a light beam in the graded-index multimode optical fiber, and n is a natural number.

6. The optical wrench according to claim 2 or 5, characterized in that: The transmittance function of the spiral zone plate satisfies the following formula: Wherein λ and f are the wavelength and focal length of the focused vortex beam, respectively, l is the waveband order of the spiral waveband plate, and r and φ are polar coordinates.

7. A method for using an all-fiber optical wrench, characterized in that: For the all-fiber optical wrench according to claim 1, the method of using the all-fiber optical wrench comprises the following steps: coupling the resonant wavelength light beam and the non-resonant wavelength light beam into the combined optical fiber in a predetermined ratio; The spiral long-period fiber grating is used to perform mode coupling on the resonant wavelength light beam transmitted in the combined optical fiber to form a first vortex light beam; The spiral zone plate is used to perform topological charge conversion on the first vortex light beam emitted from the combined optical fiber and focus it to form a first focused vortex light beam, and the non-resonant wavelength light beam emitted from the combined optical fiber is spirally phase modulated and focused to form a second focused vortex light beam, and the first focused vortex light beam and the second focused vortex light beam are applied to the particles to drive the particles to rotate.

8. The method for using the all-fiber optical wrench according to claim 7, characterized in that: When the particles are driven to rotate, the output powers of the resonant wavelength light beam and the non-resonant wavelength light beam are adjusted to control the torque generated by the first focused vortex light beam and the second focused vortex light beam on the rotating platform, thereby enabling the particles to freely switch between the three states of clockwise rotation, counterclockwise rotation and stopped rotation.

9. The method for using the all-fiber optical wrench according to claim 7 or 8, characterized in that: When the particle is driven to rotate, by controlling the on / off states of the resonant wavelength light beam and the non-resonant wavelength light beam, one of the first focused vortex light beam and the second focused vortex light beam is selectively output, thereby enabling the particle to freely switch between the three states of clockwise rotation, counterclockwise rotation and stop rotation.

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