Topological charge-wavelength adjustable vortex fiber laser based on circularly polarized light

Abstract

The invention provides a topological charge-wavelength adjustable vortex fiber laser based on circularly polarized light. The laser comprises a pump light source, a wavelength division multiplexer, a tunable filter, a first collimator, a first liquid crystal variable delayer, a metasurface, a second liquid crystal variable delayer and a polarization beam splitter which are arranged in sequence. The pump light source is used for emitting laser, the pump light source is connected with the wavelength division multiplexer, the wavelength division multiplexer is connected with the tunable filter, the tunable filter is used for adjusting the optical wavelength, and the first liquid crystal variable delayer and the second liquid crystal variable delayer are used for adjusting the topological charge of light. The first liquid crystal variable delayer converts linearly polarized light into circularly polarized light and outputs the circularly polarized light to the super-structure surface, the super-structure surface comprises a plurality of uniformly arranged super-unit cells, and each super-unit cell is provided with two isotropic super-structure surface units and two anisotropic super-structure surface units which are diagonally arranged.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber lasers, and particularly to a vortex fiber laser with adjustable topological charge - wavelength based on circularly polarized light. Background Art

[0002] Vortex beams have a spatially varying helical wavefront and carry different orbital angular momentum (OAM) values corresponding to their topological charges, showing broad application prospects in high - capacity optical communication, super - resolution imaging, and micro - manipulation.

[0003] The conventional methods for directly generating OAM beams in lasers can be generally classified into two categories: mode excitation and combination, or intracavity phase modulation. The mode excitation and combination method usually uses a mode coupler or a fiber grating to regulate the beam, while the phase modulation method is achieved by combining an intracavity spatial light modulator (SLM), a q - plate, and a metasurface. Compared with extracavity generation, the OAM beams directly generated by lasers show excellent mode purity and stability. For the research on vortex fiber lasers, the current focus is on increasing the OAM order, improving tunability, and enhancing power. And these studies have evolved from bulky and discrete solutions to more compact and integrated configurations. In all - fiber lasers, an acousto - optic mode converter can be used to quickly switch between mutually coupled OAM beams (such as l = ±1). However, due to inherent losses and crosstalk, the number of OAM modes that can effectively propagate in few - mode fibers is limited, which restricts the generation and switching of high - order spatial modes. Another method for adjusting the OAM order in solid - state or fiber lasers is the combined modulation between polarization - dependent beam - shaping devices and wave plates in the laser cavity. However, mode switching by manually or electrically rotating polarization optical elements results in slow switching times and inconsistent mode quality. Summary of the Invention

[0004] In view of this, embodiments of the present invention provide a vortex fiber laser with adjustable topological charge - wavelength based on circularly polarized light to eliminate or improve one or more defects existing in the prior art.

[0005] One aspect of the present invention provides a vortex fiber laser with adjustable topological charge - wavelength based on circularly polarized light, the laser comprising a pump light source, a wavelength - division multiplexer, a tunable filter, a first collimator, a first liquid - crystal variable retarder, a metasurface, a second liquid - crystal variable retarder, and a polarization beam splitter arranged in sequence;

[0006] The pump light source is used to emit laser light. The pump light source is connected to a wavelength division multiplexer, and the wavelength division multiplexer is connected to a tunable filter. The tunable filter is used to adjust the optical wavelength. The first liquid crystal variable retarder and the second liquid crystal variable retarder are used to adjust the topological charge number of light. The first liquid crystal variable retarder converts linearly polarized light into circularly polarized light and outputs it to the metasurface.

[0007] The metasurface includes a plurality of supercells uniformly arranged. Each supercell is provided with two isotropic metasurface units and two anisotropic metasurface units arranged diagonally. The isotropic metasurface units have the same length and width in the top view plane, and the anisotropic metasurface units have different lengths and widths in the top view plane.

[0008] With the above scheme, this patent realizes a dynamic vortex fiber laser with topological charge decoupling and wavelength tunability by embedding a liquid crystal variable retarder with broadband response and a metasurface into the laser cavity. The laser consists of multiple key components, including a 976nm laser pump light source, polarization-maintaining ytterbium-doped fiber, a tunable filter, a liquid crystal variable retarder, a metasurface, and other standard optical elements, constructing a closed ring cavity to achieve the output of an OAM beam near a wavelength of about 1030nm. The metasurface of this method processes circularly polarized light, can realize a vortex beam with a higher topological charge number, has better wavelength stability, and the high gain of the fiber laser can effectively compensate for the loss introduced by the metasurface, thus providing the possibility for the output of a high-power OAM beam. Moreover, this scheme can be electrically controlled by a liquid crystal variable retarder without rotating polarization optical elements, ensuring switching speed and mode consistency.

[0009] In some embodiments of the present invention, in the same supercell, the structures of the two isotropic metasurface units arranged diagonally are the same.

[0010] In some embodiments of the present invention, the vortex fiber laser with topological charge - wavelength tunability based on circularly polarized light further includes a second collimator and a polarization-dependent isolator connected to each other. The second collimator is connected to a polarization beam splitter. The polarization-dependent isolator is used to control the unidirectional transmission of the beam, making the optical path form a unidirectional transmission ring cavity.

[0011] In some embodiments of the present invention, the vortex fiber laser with topological charge - wavelength tunability based on circularly polarized light further includes a first lens and a second lens. The first lens is arranged between the first liquid crystal variable retarder and the metasurface, and the second lens is arranged between the second liquid crystal variable retarder and the metasurface.

[0012] In some embodiments of the present invention, the vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light further includes an arbitrary waveform generator, which is connected to a first liquid crystal variable retarder and a second liquid crystal variable retarder. The first liquid crystal variable retarder converts the light beam output by the first collimator into a left - hand circularly polarized light beam or a right - hand circularly polarized light beam based on the voltage input by the arbitrary waveform generator, and the second liquid crystal variable retarder converts the circularly polarized light output by the metasurface back into a linearly polarized light beam based on the voltage input by the arbitrary waveform generator.

[0013] In some embodiments of the present invention, the metasurface further includes a base plate, and the metasurface unit is connected to the base plate and is disposed on one surface of the base plate.

[0014] In some embodiments of the present invention, based on the positions of the supercell in two pre - set phase diagrams, two corresponding phase values are determined, and based on the two phase values, the transmission phases of two anisotropic metasurface units under x - polarization and y - polarization incidence and the rotation angles of the two anisotropic metasurface units are calculated.

[0015] In some embodiments of the present invention, in the step of calculating the transmission phases of two anisotropic metasurface units under x - polarization and y - polarization incidence and the rotation angles of the two anisotropic metasurface units based on the two phase values, the following formula is used to calculate the transmission phases of the two anisotropic metasurface units under x - polarization and y - polarization incidence:

[0016]

[0017] where respectively represent the transmission phases of the two anisotropic metasurface units under x - polarization and y - polarization incidence; E2 and E3 are both pre - set amplitude regulation values; and are respectively one of the two phase values.

[0018] In some embodiments of the present invention, in the step of calculating the transmission phases of two anisotropic metasurface units under x - polarization and y - polarization incidence and the rotation angles of the two anisotropic metasurface units based on the two phase values, the following formula is used to calculate the rotation angles of the two anisotropic metasurface units:

[0019]

[0020] where θ C and θ D respectively represent the rotation angles of the two anisotropic metasurface units.

[0021] In some embodiments of the present invention, the transmission phases of two isotropic metasurface units are calculated based on the phase values of the Gaussian beam, and the following formula is used to calculate the transmission phases of the two isotropic metasurface units:

[0022]

[0023] where E1 is a preset amplitude regulation value; i represents a complex number; represents the phase value of the Gaussian beam; and represent the transmission phases of the two isotropic metasurface units respectively.

[0024] Additional advantages, objects, and features of the present invention will be partly described below, and will become partly apparent to those of ordinary skill in the art after studying the following text, or can be learned from the practice of the present invention. The objects and other advantages of the present invention can be pointed out and obtained specifically in the specification and the drawings.

[0025] Those skilled in the art will understand that the objects and advantages that can be achieved by the present invention are not limited to the above specifically described, and the above and other objects that the present invention can achieve will be more clearly understood according to the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and do not limit the present invention.

[0027] Figure 1 is a schematic diagram of the overall architecture of a vortex fiber laser with tunable topological charge - wavelength based on circularly polarized light for this solution;

[0028] Figure 2 is a schematic diagram of the structure of the supercell for this solution;

[0029] Figure 3 is a schematic SEM diagram of the metasurface for this solution;

[0030] Figure 4 is a schematic diagram of the experimental measurement results of the vortex beam for this solution;

[0031] Figure 5 is a graph of the relationship between the output power of the OAM beam and the pump input power for this solution;

[0032] Figure 6 is a schematic diagram of the adjustable range of the output OAM beam spectrum for this solution;

[0033] Figure 7Schematic diagram of the time response of the topological charge switching of the vortex light output by the laser in this solution;

[0034] Figure 8 Schematic diagram of the phase diagram with l = 20;

[0035] Figure 9 Schematic diagram of the phase diagram with l = 2. Specific implementation manners

[0036] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in combination with the implementation manners and the drawings. Here, the illustrative implementation manners of the present invention and their descriptions are used to explain the present invention, but do not limit the present invention.

[0037] Here, it should also be noted that in order to avoid obscuring the present invention due to unnecessary details, only the structures and / or processing steps closely related to the solution of the present invention are shown in the drawings, while other details less related to the present invention are omitted.

[0038] As Figure 1 、 2 and shown in FIG. 3, the present invention provides a vortex fiber laser with adjustable topological charge - wavelength based on circularly polarized light. The laser includes a pump light source, a wavelength division multiplexer, a tunable filter, a first collimator, a first liquid crystal variable retarder, a metasurface, a second liquid crystal variable retarder, and a polarization beam splitter arranged in sequence;

[0039] The pump light source is used to emit laser light. The pump light source is connected to the wavelength division multiplexer, and the wavelength division multiplexer is connected to the tunable filter. The tunable filter is used to adjust the optical wavelength. The first liquid crystal variable retarder and the second liquid crystal variable retarder are used to adjust the topological charge of the light. The first liquid crystal variable retarder converts linearly polarized light into circularly polarized light and outputs it to the metasurface;

[0040] In the specific implementation process, the pump light source uses a 976 - nm laser pump light source.

[0041] The metasurface includes a plurality of supercells uniformly arranged. Each supercell is provided with two isotropic metasurface units and two anisotropic metasurface units arranged diagonally. The isotropic metasurface units have the same length and width in the top - view plane, and the anisotropic metasurface units have different lengths and widths in the top - view plane.

[0042] In the specific implementation process, the height of the metasurface units in the supercells is 700 nm, and the metasurface units are uniformly arranged with a period set to P = 450 nm.

[0043] In the specific implementation process, the metasurface operates near a wavelength of 1030 nm and is incident by a circularly polarized light beam. The input circularly polarized light beams can be respectively expressed as:

[0044]

[0045] In this scheme, the light beams with the same polarization output are used as two identical modulation channels, and the light beams with cross-polarization output are used as different modulation channels. The amplitude and phase of each are respectively modulated. According to the requirements of beam phase modulation and energy recycling in the laser cavity, the co-polarization output channel (LCP-LCP or RCP-RCP, where LCP-LCP means the output is an LCP beam when an LCP beam is incident, and RCP-RCP is the same) is used to generate a Gaussian beam, while the cross-polarization output channels (LCP-RCP or RCP-LCP, where LCP-RCP means the output is an RCP beam when an LCP beam is incident, and RCP-LCP is the same) are used as these two different modulation channels to generate OAM beams.

[0046] The amplitude and phase of each channel can also be adjusted accordingly. The modulation of the metasurface on the light beam is represented by the Jones matrix J, which can be expressed as:

[0047]

[0048] where E j and represent the target amplitude and phase of the three decoupled modulation channels. The left side of the formula represents the modulation of the metasurface (J) on the incident light beams with different circular polarizations (LCP or RCP), and the right side of the formula represents the LCP component and RCP component of the output after modulation.

[0049] In this scheme, the entire laser cavity uses polarization-maintaining fiber to ensure that linearly polarized light is directly obtained at the fiber output end. The pump source emits a laser with a wavelength of 976 nm and is coupled into the laser cavity with the help of a 980 / 1030 nm wavelength division multiplexer. The tunable filter (TF) is used to adjust the center wavelength of the laser output. The first collimator (Col.1) and the second collimator (Col.2) are used to connect the fiber part and the free-space optical path part; while the liquid crystal variable retarder (LCVR) is responsible for regulating the polarization direction of the light.

[0050] Adopting the above scheme, this patent realizes a dynamic vortex fiber laser with topological charge decoupling and wavelength tunability by embedding a liquid crystal variable retarder with broadband response and a metasurface into the laser cavity. The laser consists of multiple key components, including a 976 nm laser pump light source, polarization-maintaining ytterbium-doped fiber, a tunable filter, a liquid crystal variable retarder, a metasurface, and other standard optical elements, to construct a closed ring cavity for realizing the output of an OAM beam near a wavelength of about 1030 nm. The metasurface of this method is processed with circularly polarized light, which can realize vortex beams with higher topological charges, has better wavelength stability, and the high gain of the fiber laser can effectively compensate for the loss introduced by the metasurface, thus providing the possibility for the output of high-power OAM beams. Moreover, this scheme can be electrically controlled by the liquid crystal variable retarder without rotating polarization optical elements, ensuring switching speed and mode consistency.

[0051] In some embodiments of the present invention, in the same supercell, the structures of two isotropic metasurface units arranged diagonally are the same.

[0052] Specifically, in the same supercell, the length and width of two isotropic metasurface units arranged diagonally are the same when viewed from above.

[0053] In some embodiments of the present invention, the vortex fiber laser with topological charge - wavelength tunability based on circularly polarized light further includes a second collimator and a polarization-dependent isolator connected to each other. The second collimator is connected to a polarization beam splitter, and the polarization-dependent isolator is used to control the unidirectional transmission of the beam, making the optical path form a unidirectional transmission ring cavity.

[0054] In some embodiments of the present invention, the vortex fiber laser with topological charge - wavelength tunability based on circularly polarized light further includes a first lens and a second lens. The first lens is disposed between the first liquid crystal variable retarder and the metasurface, and the second lens is disposed between the second liquid crystal variable retarder and the metasurface.

[0055] In some embodiments of the present invention, the vortex fiber laser with topological charge - wavelength tunability based on circularly polarized light further includes an arbitrary waveform generator connected to the first liquid crystal variable retarder and the second liquid crystal variable retarder. Based on the voltage input by the arbitrary waveform generator, the first liquid crystal variable retarder converts the beam output by the first collimator into a left-handed circularly polarized beam or a right-handed circularly polarized beam.

[0056] To ensure the synchronous operation of two LCVRs, a single signal from an arbitrary waveform generator (AWG) acts on both devices simultaneously. This synchronous operation is crucial for maintaining the temporal coherence between the two LCVRs, and such a setup can stably and consistently achieve the desired polarization state. With a single AWG signal, the potential phase mismatch or timing error between the two LCVRs can be minimized.

[0057] Figure 5 The relationship between the output power of the laser and the pump power is shown. When LCVR1 converts the input beam into LCP, as the pump power increases to 83 mW, the laser starts to oscillate. As the pump power is further increased, a linear relationship exists between the output power of the laser and the pump power, with a slope efficiency of 6.80%. Under these conditions, the laser outputs an OAM beam with a topological charge of 2. When the beam incident on the metasurface is RCP, the corresponding laser threshold is 86 mW, and the slope efficiency is 5.02%. By adjusting the TF in the cavity, the spectral center wavelength of the OAM beam output by this laser can be flexibly varied in the range of 1014 nm to 1046 nm, as Figure 6 shown.

[0058] In the specific implementation process, this solution relies on the synergistic effect of the liquid crystal variable retarder (LCVR) and the metasurface in the laser cavity. In the entire platform architecture, the LCVR acts as a polarization controller. The y-polarized beam emitted from the collimator can be converted into a left-handed circularly polarized (LCP) or right-handed circularly polarized (RCP) beam under the action of the first liquid crystal variable retarder (LCVR1) according to the applied voltage Vp. When the applied voltage value is Vp1, the LCP beam is projected onto the metasurface. At this time, the metasurface is designed to satisfy the following functions: in the RCP output channel (LCP-RCP), an orbital angular momentum (OAM) beam with a topological charge of l = 2 is generated, while in the LCP channel (LCP-LCP), a Gaussian beam is produced. Similarly, the metasurface also satisfies the following functions: when the applied voltage is Vp2, the RCP beam shines on the metasurface, and an OAM beam with a topological charge of l = 20 is generated in the LCP channel (RCP-LCP), and a Gaussian beam appears in the RCP channel (RCP-RCP). Finally, the polarization states of the OAM beam and the Gaussian beam will be converted into the desired linear polarization state by the second liquid crystal variable retarder (LCVR2) to ensure the self-consistency of the transverse mode evolution and polarization state evolution properties of the transmitted beam in the laser.

[0059] By changing the amplitude of the voltage control signal of the first and second liquid crystal variable retarders in the cavity, the polarization control function can be quickly switched. As Figure 7As shown, the ASK modulation signal of 5 Hz is used to switch between Vp1 (3.5 V) and Vp2 (5.7 V), while the 2 kHz square wave fundamental frequency signal is used to drive the LCVR. As mentioned before, the two liquid crystal variable retarders operate under a single signal, and a diaphragm can be used to separate two OAM beams with different beam diameters. Then, the beam is focused onto a photodetector (PD) for detection after the diaphragm. The rise and fall times of the switching of vortex light with different topological charge numbers are approximately 17.5 ms and 2.4 ms respectively, which are mainly determined by the response time of the liquid crystal variable retarder. The difference between the rise and fall times is attributed to the asymmetric response characteristics of the liquid crystal material, which can be further optimized through material and device design.

[0060] Since the light of the cross-polarization component carries the phase modulation information corresponding to the vortex beam after being modulated by the metasurface, when the circularly polarized light is changed back to linearly polarized light by LCVR2, the vortex beam can be led out of the cavity with the help of a polarization beam splitter (PBS); meanwhile, the part of the optical fiber in the cavity still conducts in the Gaussian beam mode. The lens group (L.1 and L.2) focuses the light spot to a size smaller than that of the metasurface and reshapes the beam into a parallel beam again. The function of the polarization-dependent isolator (PD-ISO) is to ensure the unidirectional emission of the laser.

[0061] As mentioned before, in the system composed of the first liquid crystal variable retarder, the metasurface, and the second liquid crystal variable retarder, the polarization state changes with the voltage. In this scheme, the polarization of the OAM beam is finally converted to x polarization, and the Gaussian beam is converted to y polarization after being regulated by the second liquid crystal variable retarder, which is just adapted to the settings of the polarization beam splitter (PBS) and the second collimator.

[0062] Through the CCD camera, the intensity distribution image of the vortex beam can be directly obtained at the output end. Figure 4 (a) shows the intensity distribution and spherical wave interference pattern of the generated l = 20 vortex beam. It can be seen from this that the vortex beam has a uniform annular intensity distribution, the central dark spot corresponds to the phase singularity, and the larger the topological charge number, the larger the central dark spot; its interference pattern shows 20 spiral stripes, indicating that its topological charge number is 20. Figure 4 (b) shows the intensity distribution and spherical wave interference pattern of the l = 2 vortex beam. Compared with the l = 20 vortex beam, it has a smaller beam radius and a smaller area of the central dark spot of the beam. Its interference fringes have the same spiral direction as those of the l = 20 vortex beam, confirming that their topological charge numbers have the same sign.

[0063] In some embodiments of the present invention, the metasurface further includes a base plate, and the metasurface unit is connected to the base plate and is arranged on one surface of the base plate.

[0064] The metasurface unit is made of silicon material.

[0065] As Figure 8 and 9 shown, the phase values are on the right side of the figure. In some embodiments of the present invention, based on the positions of the supercell in two preset phase diagrams, two corresponding phase values are determined, and based on the two phase values, the transmission phases of the two anisotropic metasurface units under x-polarized and y-polarized incidence and the rotation angles of the two anisotropic metasurface units are calculated.

[0066] Specifically, by decomposing the above Jones matrix and finding its eigenvectors and eigenvalues, the responses and rotation angles of each nano-unit in the supercell that meet the above regulation requirements under linearly polarized basis incidence are obtained. For isotropic metasurface units, the light beams in the same polarization channel are regulated.

[0067] In some embodiments of the present invention, in the step of calculating the transmission phases of the two anisotropic metasurface units under x-polarized and y-polarized incidence and the rotation angles of the two anisotropic metasurface units based on the two phase values, the following formula is used to calculate the transmission phases of the two anisotropic metasurface units under x-polarized and y-polarized incidence:

[0068]

[0069] where respectively represent the transmission phases of the two anisotropic metasurface units under x-polarized and y-polarized incidence; E2 and E3 are both preset amplitude regulation values; and are respectively one of the two phase values.

[0070] In some embodiments of the present invention, in the step of calculating the transmission phases of the two anisotropic metasurface units under x-polarized and y-polarized incidence and the rotation angles of the two anisotropic metasurface units based on the two phase values, the following formula is used to calculate the rotation angles of the two anisotropic metasurface units:

[0071]

[0072] where θ C and θ D respectively represent the rotation angles of the two anisotropic metasurface units.

[0073] The rotation angle is θ in Figure 2 , where L and W are the length and width respectively.

[0074] After calculating θ C and θ DAfter that, the length and width of the anisotropic metasurface unit are determined through simulation.

[0075] In some embodiments of the present invention, the transmission phases of two isotropic metasurface units are calculated based on the phase values of the Gaussian beam, and the following formula is used to calculate the transmission phases of the two isotropic metasurface units:

[0076]

[0077] wherein, E1 is a preset amplitude modulation value; i represents a complex number; represents the phase value of the Gaussian beam; and respectively represent the transmission phases of the two isotropic metasurface units.

[0078] In this solution, considering improving the efficiency of the metasurface and controlling the cavity loss, all amplitude modulation values are set to 1, and thus a supercell composed of two identical isotropic nanorods and two identical anisotropic nanorods is constructed. The efficiency distribution of co-polarization and cross-polarization can be regulated by the number of isotropic units in the supercell. For example, a combination form such as one isotropic unit paired with three anisotropic units is adopted.

[0079] Specifically, when a beam of light irradiates the supercell, the anisotropic silicon pillars will convert the LCP irradiated on them into RCP (RCP will also be converted into LCP), while the isotropic silicon pillars will maintain the original polarization state. Therefore, by changing the proportion of the two in the supercell, the proportion of the LCP and RCP components of the outgoing beam can be changed.

[0080] The scanning electron microscopy (SEM) image of the metasurface obtained after manufacturing and processing according to the above design scheme is as Figure 3 shown.

[0081] Furthermore, compared with the scheme of directly processing linearly polarized light using a metasurface:

[0082] Using linearly polarized light for phase modulation of three polarization channels (x-polarized incident and x-polarized outgoing, y-polarized incident and y-polarized outgoing, x(y)-polarized incident and y(x)-polarized outgoing), due to energy conservation and time-reversal symmetry, for the co-polarization channel and cross-polarization channel of the outgoing beam, the amplitude modulation term and phase modulation term have the following relationship:

[0083] |T xy | = |T yx |, Φ xy = Φ yx

[0084]

[0085] In the formula, T represents the amplitude modulation of the outgoing beam, Φ represents the phase modulation, and the subscripts represent the incident polarization and the outgoing polarization (e.g., xx represents x-polarized incident and x-polarized outgoing, xy represents x-polarized incident and y-polarized outgoing).

[0086] Such coupling relationships between each phase and the amplitude response will pose obstacles in the actual design of the metasurface. For example, when designing vortex beams with different topological charges in the xx channel and the yy channel using linearly polarized light beams, it is impossible to design a completely uniform phase response in the xy (or yx) polarization channel to obtain a Gaussian beam with better quality. Additionally, in principle, the controllable degrees of freedom of a single nanorod are only three (length, width, rotation angle), while there are 6 targets for phase and amplitude modulation in the three polarization channels, making it difficult to obtain an accurate solution. By means of forced solution, the metasurface formed is more susceptible to influence on the vortex beam and the Gaussian-like beam profile obtained when incident at different wavelengths.

[0087] However, for the method adopted in this case, the 6 target values (Formula 2) of the supercell can be freely adjusted. For phase modulation: The isotropic nanorods used to form the Gaussian beam all adopt the same geometric parameter settings. When incident at different wavelengths, the phase response is also uniform. Therefore, the Gaussian beam output with the same polarization of the metasurface has better wavelength stability. For the anisotropic metasurface unit used to generate the vortex beam, it utilizes a part of the mechanism of geometric phase (i.e., the phase response is only related to the rotation angle of the structure), which also makes it have better stability when the wavelength changes, which is more conducive to implementing the wavelength tunable function of the laser. For the energy control of the same polarization and cross polarization: The amplitude modulation design using linearly polarized light is completely related to the geometric parameters of the structure. By using the supercell for control, since the target value E is set to 1, the energy of the same polarization and cross polarization is mainly controlled by the proportion of the isotropic and anisotropic unit structures in the supercell. The anisotropic metasurface unit in the specific metasurface of this scheme can adopt the structural parameters shown in Table 1 below. The phase modulation method used in this case has better phase modulation ability (reflected in the ability to easily achieve vortex beams with higher topological charges), better wavelength stability (having good phase modulation effects over a wider bandwidth, reflected in the wavelength tunable performance of the laser), and more stable efficiency modulation ability.

[0088] Table 1

[0089]

[0090] In summary, by embedding a liquid crystal variable retarder with broadband response and a metasurface into the laser cavity, the proposed solution realizes a dynamic vortex fiber laser with decoupled topological charge number and wavelength tunability. The laser consists of multiple key components, including a 976-nm laser pump source, polarization-maintaining ytterbium-doped fiber, a tunable filter, a liquid crystal variable retarder, a metasurface, and other standard optical elements, to form a closed ring cavity for achieving the output of OAM beams near a wavelength of approximately 1030 nm. The vortex beams generated by this method have flexible tunability of topological charge number and wavelength, and the high gain of the fiber laser can effectively compensate for the loss introduced by the metasurface, thus providing the possibility for the output of high-power OAM beams. In addition, the proposed laser design concept is universal. Besides the ytterbium-doped fiber laser demonstrated in this patent, this design method can also be applied to fiber lasers with other operating wavelengths, such as erbium-doped fiber lasers and thulium-doped fiber lasers. Besides being applicable to the combination of OAM topological charge numbers 2 and 20, this design method is also applicable to fiber lasers assisted by a metasurface for circular polarization multiplexing of any two OAM beams.

[0091] Those of ordinary skill in the art should understand that the various exemplary components, systems, and methods described in conjunction with the embodiments disclosed herein can be implemented in hardware, software, or a combination of both. Specifically, whether to implement in hardware or software depends on the specific application and design constraints of the technical solution. Skilled artisans can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present invention. When implemented in hardware, it can be, for example, an electronic circuit, an application-specific integrated circuit (ASIC), appropriate firmware, a plug-in, a functional card, and so on. When implemented in software, the elements of the present invention are programs or code segments used to perform the required tasks. The programs or code segments can be stored in a machine-readable medium or transmitted via a data signal carried in a carrier wave over a transmission medium or a communication link.

[0092] It should be clear that the present invention is not limited to the specific configurations and processes described above and illustrated in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and illustrated as examples. However, the method process of the present invention is not limited to the specific steps described and illustrated, and those skilled in the art can make various changes, modifications, and additions, or change the order between steps after understanding the spirit of the present invention.

[0093] In the present invention, features described and / or illustrated for one embodiment can be used in the same or a similar manner in one or more other embodiments, and / or combined with the features of other embodiments or replace the features of other embodiments.

[0094] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the embodiments of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A vortex fiber laser with adjustable topological charge number - wavelength based on circularly polarized light, characterized in that, The laser includes a pump light source, a wavelength division multiplexer, a tunable filter, a first collimator, a first liquid crystal variable retarder, a metasurface, a second liquid crystal variable retarder, and a polarization beam splitter arranged in sequence; The pump light source is used to emit laser light. The pump light source is connected to the wavelength division multiplexer, and the wavelength division multiplexer is connected to the tunable filter. The tunable filter is used to adjust the optical wavelength. The first liquid crystal variable retarder and the second liquid crystal variable retarder are used to adjust the topological charge number of light. The first liquid crystal variable retarder converts linearly polarized light into circularly polarized light and outputs it to the metasurface; The metasurface includes a plurality of supercells uniformly arranged. Each supercell is provided with two isotropic metasurface units and two anisotropic metasurface units arranged diagonally. The isotropic metasurface units have the same length and width in the top view plane, and the anisotropic metasurface units have different lengths and widths in the top view plane.

2. The vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light according to claim 1, wherein In the same supercell, the structures of the two isotropic metasurface units arranged diagonally are the same.

3. The vortex fiber laser based on circularly polarized light with adjustable topological charge number - wavelength according to claim 1, characterized in that The vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light further includes a second collimator and a polarization - dependent isolator connected to each other. The second collimator is connected to the polarization beam splitter. The polarization - dependent isolator is used to control the unidirectional transmission of the light beam, so that the optical path forms a unidirectional - transmission ring cavity.

4. The vortex fiber laser based on circularly polarized light with adjustable topological charge number - wavelength according to claim 1, characterized in that, The vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light further includes a first lens and a second lens. The first lens is arranged between the first liquid crystal variable retarder and the metasurface, and the second lens is arranged between the second liquid crystal variable retarder and the metasurface.

5. The vortex fiber laser based on circularly polarized light with adjustable topological charge number - wavelength according to claim 1, characterized in that, The vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light further includes an arbitrary waveform generator. The arbitrary waveform generator is connected to the first liquid crystal variable retarder and the second liquid crystal variable retarder. Based on the voltage input by the arbitrary waveform generator, the first liquid crystal variable retarder converts the light beam output by the first collimator into a left - hand circularly polarized light beam or a right - hand circularly polarized light beam, and the second liquid crystal variable retarder converts the circularly polarized light output by the metasurface back into a linearly polarized light beam based on the voltage input by the arbitrary waveform generator.

6. The vortex fiber laser based on circularly polarized light with tunable topological charge number - wavelength according to claim 1, wherein The metasurface further includes a base plate. The metasurface unit is connected to the base plate and is arranged on one surface of the base plate.

7. The vortex fiber laser based on circularly polarized light with adjustable topological charge number - wavelength according to claim 1, characterized in that, Based on the positions of the supercells in two pre - set phase diagrams, two corresponding phase values are determined. Based on the two phase values, the transmission phases of the two anisotropic metasurface units under x - polarization and y - polarization incidence and the rotation angles of the two anisotropic metasurface units are calculated.

8. The vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light according to claim 7, characterized in that, In the step of calculating the transmission phases of the two anisotropic metasurface units under x - polarization and y - polarization incidence and the rotation angles of the two anisotropic metasurface units based on the two phase values, the following formula is used to calculate the transmission phases of the two anisotropic metasurface units under x - polarization and y - polarization incidence: wherein, respectively represent the transmission phases of two metasurface units with anisotropy under x-polarized and y-polarized incidence; E2 and E3 respectively represent the preset amplitude regulation values; and are respectively one of the two phase values.

9. The vortex fiber laser based on circularly polarized light with adjustable topological charge number - wavelength according to claim 7, characterized in that, In the step of calculating the transmission phases of two anisotropic metasurface units under x-polarized and y-polarized incidence and the rotation angles of the two anisotropic metasurface units based on two phase values, the following formula is used to calculate the rotation angles of the two anisotropic metasurface units: where, θ C and θ D represent the rotation angles of two anisotropic metasurface units, respectively.

10. The vortex fiber laser with tunable topological charge number - wavelength based on circularly polarized light according to claim 7, characterized in that, Based on the phase values of the Gaussian beam, the transmission phases of two isotropic metasurface units are calculated, and the following formula is used to calculate the transmission phases of the two isotropic metasurface units: Among them, E1 is a preset amplitude regulation value; represents the phase value of the Gaussian beam; and respectively represent the transmission phases of two isotropic metasurface units.

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