1. Introduction

Due to the compactness and flexibility, metasurfaces have emerged as a promising tool to manipulate the amplitude, phase, and polarization state of light in a highly controllable manner, enabling a wide range of applications such as sensing [1], imaging [2,3], holography [4], and structural colouration [5,6]. In particular, gap-surface plasmon (GSP) metasurfaces, which are sandwich structures consisting of metal nanoparticles, a thin dielectric layer, and a metal film of optical thickness from top to bottom, have shown remarkable performances in enhancing light-matter interactions and creating highly localized and intense optical fields. It has been confirmed that GSP metasurfaces with cavity-like Fabry-Pérot (FP) resonance can achieve high conversion efficiency exceeding 80% [7–12].

Recently, orbital angular momentum (OAM) beams have attracted significant attention in laser optics due to their unique properties, such as helical wavefronts and carrying topological charges l [13,14], which can be utilized in various fields, such as high-capacity optical communication [15,16], super-resolution imaging [17,18], and optical manipulation [19,20]. In comparison to external generation of OAM beams with respect to a laser cavity [21–24], generation of OAM beams directly inside the cavity offers advantages such as compact setup and user-friendliness [25,26].

By inserting a plasmonic/dielectric metasurface into a solid-state laser, the manipulation versatility of the intracavity metasurfaces over light wave has been demonstrated, leading to OAM beams with high mode purity and customizable spatial mode profiles [27–30]. Compared to solid-state lasers, fiber lasers offer advantages such as no need for water cooling (or simple scheme for cooling), high efficiency, high gain, and low cost. Furthermore, LP₀₁ mode is the fundamental mode of light propagation in single-mode fibers (SMFs), which is almost equivalent to a Gaussian beam in free space. The existing clear and pure LP₀₁ mode in SMFs provides an ideal prerequisite for metasurfaces to generate OAM structured light [31,32]. Our previous demonstration of a polarization-maintaining vortex fiber laser with GSP metasurfaces has verified the mode conversion principle and high mode purity emission [32]. However, the laser resonator is limited to linear cavity, and all fiber components are packaged with polarization-maintaining fibers with high cost, which to some extent, hinders its practical widespread applications. Vortex lasers with ring cavities or other structures using non-polarization-maintaining fibers could provide more choices to meet demands of different users. Meanwhile, wavelength-tunable characteristics is one of the most valuable functions of structured light lasers [33], and wide wavelength tunability greatly facilitates their applications in optical communications and manufacturing. OAM fiber lasers based on conventional mode conversion devices (such as spatial light modulators [34], long-period fiber gratings [35,36], and mode selective couplers [37]) have a tuning range of approximately 3 nm to 35 nm. It has been demonstrated that the GSP metasurfaces can generate OAM beams exceeding 100 nm operation bandwidth [11,32]. As the intracavity modulation components, GSP metasurfaces hold promise for achieving a wider tunable wavelength range of the emitted OAM beams, compared to the aforementioned mode conversion components.

In this paper, we experimentally demonstrate a wavelength-tunable ytterbium-doped fiber (YDF) laser with intracavity GSP metasurfaces and non-polarization-maintaining single-mode fibers, which enables direct generation of high-purity OAM beams from 1020 nm to 1060 nm. Four GSP metasurfaces with different orthogonal linear-polarization conversion efficiencies and/or different topological charges were designed and fabricated. The vortex fiber laser exhibits figure-9 structure, which matches the reflection working style of the metasurfaces. To ensure the balance between sufficient co-polarized light components for feedback oscillation within the laser cavity and efficient OAM light output on the cross-polarization channel, we explored the impact of different cross-polarization conversion ratios of the metasurfaces on the output characteristics of the laser. A wavelength-tunable filter is included in the cavity to continuously tune the operation wavelength of the laser oscillation. By these approaches we achieved OAM beams with switchable topological charge (l = + 1 or l = −1) and tunable emission wavelength (40 nm span), as well as changeable lasing threshold or slope efficiency of the resonator. The scheme unleashes potential for generation of higher-order OAM modes, as well as for direct integration of the metasurfaces onto fiber endfaces [38,39], enabling more compact system strategies to significantly enhance the practical value of manipulating structured beams at laser sources.

2. Design and results

The work principle of the GSP metasurface utilizing a metal-dielectric-metal structure with orthogonal linear-polarization conversion is illustrated in Fig. 1(a). The GSP metasurface efficiently converts Gaussian beams to OAM beams on the cross-polarized channels of the incident light with controllable conversion efficiency. GSP metasurfaces typically operate in reflection mode, and thus we propose a figure-9 (F9) fiber laser cavity configuration, as shown in Fig. 1(b). The resonator uses standard non-polarization-maintaining single-mode fibers (gray lines). The 976 nm continuous-wave pump light is coupled into the laser cavity through a 976/1030 nm wavelength division multiplexer (WDM). The YDF provides the required gain for laser oscillation. The optical isolator (ISO) ensures unidirectional propagation of light. Following the three-terminal circulator (Cir.) with a defined transmission trajectory (indicated by arrows), the Gaussian beam with a 1-mm waist radius is coupled into the free space through a collimator (Col.). The outgoing Gaussian beam passes through a polarization-sensitive beam splitter (PBS) with minimal loss (x-pol. state) by rotating the polarization controller (PC). Subsequently, the beam is focused onto the metasurface (MS) using a 50-mm focal length achromatic lens (L.). The reflected beam from the metasurface comprises both the partially converted OAM beam on the cross-polarization channel and the unconverted residual beam on the co-polarization channel. The OAM beam is coupled outside the cavity via the PBS, while the residual beam is coupled back into the fiber by the collimator after passing straight through the PBS, achieving self-consistent transverse mode in the F9 laser cavity. The transformation from Gaussian to OAM modes also applies to a y-polarized incident Gaussian mode case but results in the generation of an x-polarized OAM mode, and the y-polarized residual beam will stay in the cavity to form the laser resonance. In this case, the emitted OAM beam aligns with the transmission channel of the PBS, while the residual beam corresponds to its reflection channel. The metasurface is capable of generating OAM beams over a broad range of wavelengths (see its broadband characteristics later). Therefore, a wavelength-tunable filter (TF, red dashed box) is incorporated in the cavity to enable continuous adjustment of the operation wavelength of the laser oscillation.

 

Fig. 1. Design of the OAM fiber laser. (a) Schematic of the gap-surface plasmon metasurface for generating OAM beams. Inset graph: the configuration of the metasurface unit cell. (b) Schematic of the configuration of the metasurface-assisted OAM figure-9 fiber laser. The blue and green arrows indicate the linear polarization of light along the x and y directions respectively.

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The GSP metasurface is comprised of gold (Au) nanobricks, a SiO₂ spacer, and an Au film at the bottom, with the Au nanorods oriented at a 45° angle with respect to the x-axis [Fig. 1(a)]. The thicknesses of the three layers, from top to bottom, are 80 nm, 110 nm, and 130 nm, respectively. The distances between two adjacent unit cells are 550 nm in both x and y directions. Leveraging orthogonal linear-polarization conversion, the reflected light from the metasurface contains a cross-polarized light component [40]. Besides the plasmon resonance generated by the Au nanobricks, the Au film introduces cavity-like FP resonance. Compared to single layer plasmonic metasurfaces, GSP metasurfaces exhibit significantly improved polarization conversion efficiencies. However, the cross-polarized light component is considered as a loss channel in the laser cavity, and an excessively high polarization conversion ratio can substantially increase the threshold pump power of the laser. To ensure sufficient co-polarized light components for feedback oscillation within the laser cavity and efficient OAM light output on the cross-polarization channel, a balanced design is necessary.

To explore the impact of different cross-polarization conversion ratios on the output characteristics of the laser, we utilize the finite element method to calculate the reflectivity and phase shift of the metasurface unit cell at the wavelength of 1030 nm. Figures 2(a) and 2(b) display four different sets of nanorod sizes we selected at 20% and 40% cross-polarized reflectivity (R_cr). The adjacent unit cells exhibit a cross-polarized phase shift ($\mathrm{\varphi }$_cr) increment of π/4, as well as nearly identical co-polarized (R_co) and cross-polarized reflectivities. To realize a full phase modulation of 2π, we added to eight sets of unit cells by rotating the existing four nanobricks by 90°, as the phase shift difference between nanobricks with a rotation angle of 90° is π while maintaining the same amplitude [40]. Subsequently, by arranging the metasurface unit cells clockwise or counterclockwise with a phase shift increment of π/4, we designed four metasurfaces, named as MS1 (R_cr = 20%, and l = 1), MS2 (R_cr = 20%, and l = −1), MS3 (R_cr = 40%, and l = 1), and MS4 (R_cr = 40%, and l = −1) respectively. They were then fabricated utilizing single-step electron-beam lithography, with the SEM images shown in Fig. 2(c). To ensure that the beam impinging on the metasurface from the fiber laser cavity is a Gaussian mode, we tracked the evolution of the co-polarized residual beam along the standard SMF, since the original residual beam does not exhibit the same intensity distribution as a perfect Gaussian beam. Figures 2(d) and 2(e) present the simulated and measured intensity of the co-polarized residual beam reflected from MS3, respectively. The intensity was recorded again after the residual beam was coupled into the single-mode fiber through the collimator and propagating for a length of 4.4 meters (several times shorter than the fiber length of the resonator). The beam transformed back to the perfect LP₀₁ mode, almost equivalent to a Gaussian mode in free space, as shown in Fig. 2(f), due to the mode filtering effect of the single-mode fiber. The measured conversion efficiency of the residual beam evolving into the perfect LP₀₁ mode is approximately 30%, and these losses are still acceptable for the laser operation.

 

Fig. 2. Simulation and measurement of the metasurfaces. The simulated co-polarized reflectivity (R_co), cross-polarized reflectivity (R_cr) and the cross-polarized phase shift ($\mathrm{\varphi }$_cr) of the metasurfaces designed for (a) R_cr = 20% and (b) R_cr = 40%. (c) Scanning electron microscopy (SEM) images of the fabricated metasurfaces corresponding to the following properties: MS1 for R_cr = 20% and l = 1, MS2 for R_cr = 20% and l = −1, MS3 for R_cr = 40% and l = 1, and MS4 for R_cr = 40% and l = −1. The simulated (d) and measured (e) intensities of co-polarized residual beam components reflected from the metasurface MS3. (f) Measured intensity of the co-polarized residual beam converted to a perfect Gaussian beam after being coupled into the single-mode fiber and propagating for 4.4-m in the fiber.

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Afterwards, the GSP metasurfaces was inserted into the F9 fiber laser without the tunable filter, as illustrated in Fig. 1(b). The vortex laser emits at the central wavelength of 1035 nm. Figure 3(a) experimentally demonstrates the intracavity generation of 1^st and −1^st order OAM beams. In the first row of Fig. 3(a), transverse intensity distributions of the emitted donut-shaped OAM beams are depicted, wherein the central intensity minimum arises due to the phase singularity resulting from the helical envelope of the OAM beam. The second row of Fig. 3(a) shows the fork-shaped interference fringes, formed by interfering them with Gaussian beams utilizing a home-built Michelson interferometer. The opening directions of the fork-shaped patterns are opposite for topological charges with opposite signs. The order of the OAM beam can be deduced from the number of displaced fringes in the interference pattern, which further confirms the effectiveness of our design.

 

Fig. 3. Output characteristics of the OAM fiber laser. (a) Measured intensity distribution of the OAM beams emitted from the cavity based on MS1∼MS4 (the first row), and the interference fringes of the OAM beams when interacting with Gaussian beams (the second row). (b) Output power versus pump power of the vortex laser. Inset graph: the detailed zoom-in view of the dotted frame.

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Figure 3(b) shows the relationship between the output power of the vortex beam and the injected pump power. Since the cross-polarization component by the GSP metasurface corresponds to loss for lasing, insertion of a metasurface with 40% cross-polarization reflectivity means that a higher pump injection fluence is required to provide the gain up to the threshold condition, in comparison to the counterpart with 20% cross-polarization reflectivity. Indeed, the MS3 and MS4 incorporated situations need relatively higher threshold pump power, (63 mW versus 60 mW). On the other hand, OAM beam of higher power can be extracted if employing a metasurface with larger cross-polarization conversion. Consequently, the MS3 and MS4 based lasers exhibit a higher slope efficiency (∼4.8%), while the MS1 and MS2 based lasers exhibit a lower slope efficiency (∼2.2%). Considering the slightly different lasing threshold, one could design in the future a metasurface with R_cr larger than 40%, in order to enlarge OAM beam power and the slope efficiency further.

The OAM mode purity can be quantitatively evaluated by demodulating the emitted OAM beams using a spatial light modulator (SLM) that encodes spiral phases with different orders. The similar characterization method as mentioned in Ref. [32,41] was utilized. As shown in Fig. 4(a), the spiral phases with −3 to +3 OAM order were loaded onto the SLM, and the demodulated beam was then projected onto a CCD camera. The bright central spots occur where the SLM orders are opposite to that of the emitted OAM beams (indicated by the red box). The OAM mode purity is determined as the power weight of the relative central intensity of the beam after demodulation by the helical phase with opposite topological charge, where the relative central intensity is obtained by subtracting the average edge background intensity from the average central intensity. Figure 4(b) confirms the high mode purities (above 90%) of the emitted OAM beams from the laser cavity based on the four GSP metasurfaces.

 

Fig. 4. OAM mode purity characterization. (a) Demodulation and characterization of the emitted OAM beam by the SLM. (b) Measured OAM mode purities based on the GSP metasurfaces inside the laser cavity.

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The designed GSP metasurfaces enable manipulation of the light wave over a wide spectral range, making it possible to achieve wavelength-tunable output of OAM beams in the fiber laser. Figure 5(a) depicts the simulated variation of the cross-polarized phase shift φ_cr versus wavelength (1000 nm to 1100 nm) for selected metasurface unit cells with R_cr = 40%. The cross-polarized phase shift increment Δφ_cr of π/4 is maintained between adjacent unit cells at specific wavelength, resulting in a total phase shift increment of 2π, which is essential for generation of OAM beams in the broadband wavelength range. Figure 5(b) shows the simulated operation efficiency for both co- and cross- channels from 1000 nm to 1100 nm (take the MS3 as an example), and the relatively stable reflection (roughly, 30% R_cr and 55% R_co) can be maintained within the range of 100 nm. The central wavelength of the tunable filter can be switched to continuously tune the wavelength of the OAM beam emitted from the fiber laser. Using the metasurfaces MS3 and MS4 as examples, the output center wavelength of the OAM fiber laser spans from 1020 nm to 1060 nm, as shown in Fig. 5(c). Compared with the tuning range of OAM fiber lasers based on conventional mode conversion components (typically, from 3 nm to 35 nm) [34–37], our metasurface-assisted fiber laser demonstrates a larger tuning range (40 nm). The wavelength tuning capability of the laser is limited not only by the gain spectrum of the single-mode Yb-doped fiber but also by the effective operating wavelength and the efficiencies of other intracavity components such as isolator, collimator, etc. Hence the narrower tunable range compared to the potential of the GSP metasurfaces is reasonable. The tunable wavelength can be expanded by reducing the overall losses within the cavity in wider wavelength range. For example, the broadband and efficient dielectric metasurfaces can be designed to simplify the intracavity configuration, and losses in the cavity can be reduced by minimizing the number of intracavity components in the future. Additionally, fluctuations in the pump power necessary for the laser to achieve stable oscillation conditions arise from the variations in overall operating efficiency of intracavity components with wavelength. Figure 5(d) demonstrates that the laser exhibits largest intracavity net gain close to 1040 nm, while highest pump injection is needed at 1060 nm to reach the oscillation threshold. Notably, changes of the operation wavelengths do not affect the mode purity of the emitted OAM beams (all achieving ∼90%). The intensity distribution of the OAM beams from 1020 nm to 1060 nm and the fork-shaped interference fringes are depicted in Figs. 5(e) and 5(f), further confirming that our designed laser effectively achieves intracavity output of OAM beams over a broadband range.

 

Fig. 5. Broadband tunable characteristics of the OAM fiber laser. (a) Simulated cross-polarized phase shift (φ_cr) of the metasurface unit cells with R_cr = 40% from 1000 nm to 1100 nm. (b) Simulated reflectivity of the MS3 from 1000 nm to 1100 nm. (c) Tunable optical spectrum of the OAM wavelength-tunable laser from 1020 nm to 1060 nm. (d) Threshold pump power (blue dots) and OAM mode purity (red dots) at different operation wavelengths. (e), (f) Measured intensity distribution (first row) and interference patterns (second row) with Gaussian beams of the output OAM beams from 1020 nm to 1060 nm. In (e) MS3 is used and in (f) MS4 is used.

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3. Conclusion

In summary, we have demonstrated a wavelength-tunable figure-9 fiber laser that integrates GSP metasurfaces for efficient and stable generation of ±1st-order OAM beams with high mode purity (up to 93%). Thanks to the broadband and efficient beam manipulation capabilities of the GSP metasurfaces, we can generate OAM beams ranging from 1020 nm to 1060 nm from the laser resonator, wider than other traditional mode conversion methods. The figure-9 cavity design broadens the scope of the existing OAM fiber laser with metasurfaces, taking both the cost (all using non-polarization-maintaining fibers) and work efficiency into account, and may provide more approaches for light control in different domains [42]. Furthermore, our scheme can be easily extended to generation of higher-order OAM beams or more complex structured beams. In the future, polarization multiplexing metasurfaces or active metasurfaces can be utilized to enable reconfigurable modulation of structured beams inside the cavity [43–46], thereby opening possibilities for manipulating structured beams at the laser sources nearly at will.