The generation and delivery of various laser beams have attracted wide attention. These developments have been promoted by the potential application of laser technology in key fields such as medical treatments^[1], communications^[2], and material processing^[3]. Among them and of particular interest are the 980 nm lasers, which serve as a vital pump light source for amplifiers and fiber lasers^[4]. Additionally, single-mode 980 nm lasers are widely used in laser surgery and cancer therapy^[5], thanks to the fact that the absorption peak of water molecules is located at 980 nm, resulting in a small penetration depth of the tissue^[6]. Meanwhile, these band lasers can be applied for material processing and engineering^[7]. For these applications, enhanced laser beam quality and stability are paramount. However, the majority of current 980 nm fiber lasers encounter significant challenges in achieving single-mode output due to the influence of multimode oscillation. This limitation prevents them from meeting the high beam quality requirements essential for the mentioned applications^[6]. As the output power increases, high-power lasers tend to exhibit multi-mode output, leading to a degradation in beam quality and compromised laser stability. Meanwhile, high-power lasers in the conventional silica-core fibers have to suffer nonlinear effects, which can be weakened via the hollow-core fiber (HCF) with an air light-guiding channel^[8,9].

The micro-structured cladding in the HCF effectively confines the majority of the light energy within the air core^[10]. The HCF has excellent beam propagation performance such as flat dispersion^[11], high damage threshold^[12], and ultra-low loss^[13]. This makes it possible to allow the transmission of average and peak powers that exceed the limitations of silica-core fiber without nonlinear effects^[14]. Among them, the anti-resonant hollow-core fiber (ARF) exhibits lower transmission loss and wider bandwidth due to its unique light-guiding mechanism^[15]. Meanwhile, the mode number and mode-field distributions of the laser beam can be controlled by optimizing the coupling relationship between the core and cladding modes^[16,17]. As such, ARF demonstrates a remarkable ability to control modes and shape the mode-field distribution due to its structural flexibility, enabling a high degree of customization of the mode field^[18,19]. However, the transmission loss of the ARF with the nested cladding structure over long distances is lower, which means that both losses of the fundamental mode and higher-order modes (HOMs) are low^[20]. As such, HOMs cannot be filtered out in short distances, which further limits their application because they require compact size, high beam quality, flat dispersion, and precise mode-field distributions^[21]. Meanwhile, cascaded devices feature a complex structure and lower stability^[22]. Consequently, the elevated requirements involve effectively HOMs over short distances and enhancing beam quality in 980 nm. This can be realized through the implementation of more flexible and efficient structural designs.

In this paper, we propose a laser beam mode shaper based on a novel heptagonal core ARF. The mode shaper can filter out HOMs that rely on the cladding nodes within a given length. We conduct a numerical investigation and simulation of the geometric changes of the cladding nodes and obtain the influence rules of the node geometry for the confinement loss of the ARF. Meanwhile, the structural parameters of the cladding nodes are further optimized using a finite element mode solver. The ability and stability of the mode shaper to control modes are verified by comparing the beam quality of the light source and the output of the ARF. The mode shaper breaks the inherent design concept and can be applied to fiber mode filtering and mode conversion in compact system applications.

The proposed heptagonal core ARF structure is shown in Fig. 1. The thin walls surrounding the core act as anti-resonant units, which can confine the energy to the air core due to destructive interference. The fiber features seven cojoined cladding tubes that form seven cladding nodes, which enhances the coupling between the core and cladding modes and improves single-mode transmission characteristics^[23]. The diameter of the air core, denoted as $D_c$, is defined as the diameter of the inscribed circle within the central air region. The tube wall thickness is $t$. The geometrical parameters of the cladding nodes are denoted as the radius of the node’s curvature $R_b$ and $R_s$, respectively. The cladding nodes can enhance the coupling between higher-order core modes and cladding modes, thereby enabling the filtering of the HOMs.

As the wall thickness of the cladding tubes varies, the resonant band of the MAF changes accordingly. The relationship between the cladding tube and the central wavelength of the anti-resonant band can be expressed as^[24]$$t=\frac{(m-0.5){\lambda }_0}{2\sqrt{n^2-1}},$$(1)where $m$ equals one positive integer, $n$ is the refractive index of the quartz, and ${\lambda }_0$ is the central wavelength. To enhance the stability of the fiber microstructure, $t$ is designed to ensure ${\lambda }_0=980\mathrm{nm}$ is located in the second anti-resonant window to reduce the ARF fabrication difficulty. Wall thickness $t$ is set as 0.7 µm, and the core diameter $D_c$ is 30 µm, equaling 30 times the operating wavelength to minimize the number of higher-order modes^[20,25]. The second anti-resonance window, calculated using the resonance formula, is in the range of 725–1445 nm.

The finite element mode solver (COMSOL Multiphysics) is used to calculate and analyze the proposed ARF structure. In simulation, the mesh size is optimized by setting $\lambda /6$ and $\lambda /4$ for the glass regions and air regions, respectively. A perfectly matched layer is applied^[26]. The simulation results show that the first- and second-order resonance peaks are located at 1.46 and 0.72 µm. This is consistent with the theoretical calculations, confirming that the guiding mechanism is based on the anti-resonance mechanism. Figure 2(a) illustrates the smoothed loss spectra for the fundamental mode and the first-order HOM, accompanied by their corresponding mode-field distributions. The ${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{11}$ at 800–1300 nm follow the same trend due to the same anti-resonant window. The loss of the HOMs at 980 nm is observed to be 31.4 dB/m, thus ensuring to filter out the HOMs within a short length. Figure 2(b) shows the confinement loss of the ARF as the geometry of the cladding nodes varies. The optimal confinement loss is achieved when the node’s curvature $R_b=1\mathrm{\mu m}$ and $R_s=0.1\mathrm{\mu m}$. Theoretical analysis reveals that optimizing the shape of the cladding nodes introduces lower transmission loss^[27]. Meanwhile, the calculated results verify the filtering capability of the HOMs due to the high HOM extinction ratio. Consequently, the designed ARF can realize short-distance single-mode transmission of 980 nm lasers, enabling multi-mode laser beam shaping.

The ARF is fabricated using the stack-drawing technique and gas-inert pressurization control, which is utilized to regulate fiber structure and thereby enhance single-mode transmission characteristics^[28]. The increased diameter of the cladding tubes and the applied pressure facilitate cladding node formation, leading to a heptagonal core shaped by viscous resistance and surface tension at temperatures ranging from 2000°C to 2080°C. Inert gas is employed in two distinct zones, corresponding to the core and cladding tubes, with pressure in each area being independently regulated^[29]. The pressure difference between the inside and outside cladding tubes is controlled to prevent the collapse of internal cladding tubes and to optimize the cladding nodes’ geometry in real time. As such, the optimized control facilitates positioning the central band of the fabricated ARF within the second anti-resonant window and reducing the transmission loss^[28,30]. As a result, Figs. 3(a) and 3(b) show the scanning electron microscope (SEM) image of the ARF, which exhibits a core diameter ($D_c$) of 27.8 µm with a wall thickness ($t$) of 0.69 µm. The nodes’ curvatures $R_b$ and $R_s$ are 1.5 and 0.4 µm, respectively.

A supercontinuum light source (SC, YSL, SC-5) is adopted to characterize the transmission and loss performance of the fabricated ARF. The optical receiver utilizes the optical spectrum analyzer (OSA, Yokogawa, AQ6370D). Light is coupled from the SC to the fiber under test (FUT), and then the output is received by the OSA, both of which are coupled through the plano-convex lens and dielectric mirrors. The ARF is fixed on the piezo-actuated stages (Thorlabs, MAX302), and a cut-back measurement is performed from 2 to 0.3 m. The dependability of the measurements is ensured by obtaining spectra with less than a 6% variation in three separate instances. Figure 4(a) displays transmission spectra for long fiber lengths (blue curve) and short fiber lengths (red curve), as observed from the experimental data. The transmission band ranges from approximately 800–1100 nm and 1200–1300 nm. Figure 4(b) depicts the measurement loss spectrum in the 900–1060 nm range, which is located in the second-order anti-resonance window of the ARF. The inset figure shows the measurement loss spectra in the 800–1300 nm range. The loss of the fabricated ARF at 980 nm is observed to be 1.5 dB/m. The presence of the cladding nodes can introduce Fano resonances, leading to excessive loss in the ARF and sharp peaks in the loss spectrum.

Compared to the simulated loss spectrum of the fundamental mode delineated in the orange curve, the transmission loss has the same trend. Nevertheless, the loss of the ARF exhibits potential for further optimization due to the differences in the structural parameters between fabrication and simulation. Meanwhile, modification in the geometry of cladding nodes can efficiently reduce the loss of fundamental mode while concurrently enhancing the coupling interaction between air high-order modes and dielectric modes, resulting in an elevated loss for HOMs. Consequently, optimization of these cladding node shapes and structural parameters in the ARF design is instrumental for enhancing the fundamental mode content and the HOM extinction ratio. This makes it possible for the beam to maintain single mode and possess near-diffraction-limited quality.

The 1-m-long ARF is employed for the laser beam mode shaper to realize mode shaping and filtering, and the schematic diagram of the laser beam mode shaper is shown in Fig. 5. A 980 nm continuous-wave laser source is used as the input, and its beam quality is 1.56. A collimated beam is obtained by transmitting a 980 nm laser source through a plano-convex lens with a focal length of 50 mm, achieving a transmittance exceeding 90%. Pellicle beam splitter (PBS) 1 splits 8% of the power to power meter (PM) 1 to calibrate fluctuations in the power injected into the ARF, while PBS 2 directs 92% to PM 2 to monitor the output power. A plano-convex lens with a focal length of 50 mm can effectively focus the collimated beam onto the core of the ARF. The FUT with a diameter of 25 cm is placed horizontally on the $XYZ$-stages to minimize bending losses^[31], and the coupling position can be optimized by adjusting the positions of the $XYZ$-stage. The inset figure shows the near-field intensity pattern at the output of the ARF. The remaining laser power from PBS 2 is used to measure the beam quality and relies on the measurement systems ($M^2$, Thorlabs, M2MS-BP209IR2/M). At the ARF output, the output beam is collimated by a plano-convex lens with a focal length of 100 mm. Two dielectric mirrors are used to optimize the collimated beam, which can ensure the beam meets the $M^2$ system measurement standard. Then, a plano-convex lens with a focal length of 250 mm is used to form a focused beam for the measurement system. The $M^2$ measurement is performed following the method described in the ISO/TR 11146-3:2004 standard^[32].

The beam-shaping capability of fabricated ARF is verified by assessing alterations in the beam quality of both the incident light source and the output light from the ARF. Figure 6 illustrates the enhancement of the 980 nm laser beam quality achieved by the fabricated ARF, which is monitored using the $M^2$ measurement systems equipped with a scanning-slit beam profiler (Thorlabs B209-VIS/M). Figure 6(a) shows that the beam quality, represented by $M^2$, is measured as 1.56 with a transmission power of 40 mW. The Gaussian fitting curve of the beam intensity in Fig. 6(b) is obtained by fitting the beam spot in both the $x$- and $y$-directions using a Gaussian function. This approach facilitates a more convenient measurement of the full width at $1/e^2$. In Fig. 6(b), the Gaussian fitting curve of the beam intensity reveals a full width at $1/e^2$ of 1887 nm. Among them, the horizontal axis represents the position of the fitting curves in the $x$- and $y$-directions. This indicates the relative position of the beam within the entire $M^2$ system measurement plane and is not related to beam quality. The mode-field distribution of the light source is shown in the inset of Fig. 6(b). The center of the peak energy (green cross) does not overlap with the center of the mode-field distribution (blue cross). This indicates that the beam poorly fits the Gaussian curve, as also evidenced in Fig. 6(b). Then, by incorporating the ARF into the system and adjusting the position of the ARF end-face, the lens can improve the coupling efficiency of the laser transmission. Figure 6(c) shows that the beam quality is improved to 1.03 after transmitting through a 1-m ARF. This indicates a $\sim 30\%$ enhancement in the output beam quality and approaches the diffraction limit. The full width at $1/e^2$ calculated by the Gaussian fitting curve is 874 nm in Fig. 6(d). The inset of Fig. 6(d) is the mode-field distribution of the output after the ARF. Comparing mode-field distributions and Gaussian fitting curves in Figs. 6(b) and 6(d), it can be observed that the mode exhibits a better single-mode state after passing through the 1-m ARF fabricated herein.

Consequently, as a laser beam mode shaper, the ARF can filter out HOMs of the laser and effectively optimize mode-field distributions to achieve single-mode laser outputs within a short distance. Meanwhile, the beam mode shaping capability can be further enhanced by optimizing the structure to increase the HOM extinction ratio and reduce the fundamental mode loss.

While verifying the good single-mode transmission characteristics of the self-made ARF, the laser input power is adjusted, and the transmission efficiency and coupling efficiency are tested. Transmission efficiency (TE) is the ratio of the output power to input power, defined by $\mathrm{TE}=P_o/P_i$^[33], where $P_o$ denotes the output power measured after L3, while the input power $P_i$ is measured before L2. Due to the confinement loss of the ARF and the existence of the cladding nodes, the transmission efficiency is about 60%. Figure 7 depicts the measured laser output power after transmission through the 1-m ARF. The red curve is its corresponding coupling efficiency $\eta$, which is defined as the efficiency excluding the transmission loss, and is expressed as^[33]$$\eta =\frac{P_o}{P_i}·{10}^{\frac{\alpha L}{10}},$$(2)where $\alpha$ is the measured fiber transmission loss (unit of dB/m), and $L$ is the transmission length. The estimated coupling efficiency is approximately 85%. Among them, a 10% loss may occur due to minor shifts in the fiber coupling position, resulting in the mismatch of the mode field and some light that transmits in the cladding region and nodes with higher loss^[34]. By adjusting the coupling position in real time and optimizing the experimental platform, the coupling efficiency can be improved, considering environmental disturbances and losses from lenses and reflectors.

To ensure the stability of the laser beam mode shaper, the output power of the ARF is measured continuously for one hour, by setting the laser power as 600 mW. The output power and its corresponding beam quality are tested during the experiment every five minutes. The results are presented in Fig. 8(a). A marginal decrease in the output optical power is observed, which can be negligible. The slight decrease in power is attributed to environmental factors rather than structural damage to the ARF. The fiber structure remains undamaged during prolonged operation. Any deviations in the coupling position can be compensated for by adjusting the $XYZ$-stages, although manual adjustment of the coupling position is not performed in the experiment. Since the shaper can stably transmit 600 mW of optical power for 60 min, the relationship between $M^2$ and time is measured at this power level. The beam quality remains relatively stable at 1.04 throughout the one-hour test period. Figures 8(b) and 8(c) illustrate the beam quality results at 0 and 60 min. The $M^2$ is 1.03 at 0 min and increases to 1.04 due to deviations in the coupling position. The measured beam profile exhibits minimal and negligible changes. The results indicate that the laser beam mode shaper can optimize the beam quality, realize mode shaping, and operate stably over extended periods.

In summary, we have successfully fabricated and experimentally characterized a laser beam mode shaper based on the heptagonal core ARF suitable for 980 nm transmission. The wall thickness of the ARF is controlled at the micron level, achieving well-light-guiding properties in the near-infrared bands. The ARF can be used for the laser beam mode shaper over short lengths due to the unique fiber structure and the presence of cladding nodes. After continuous-wave laser beam delivery through a 1-m ARF, the beam quality improves by 30%. The output light is in the fundamental mode of the ARF at 980 nm, exhibiting near-diffraction-limited beam quality. In addition, the laser beam mode shaper demonstrates excellent stability in both power and beam quality when operating at a power level of 600 mW for one hour. Furthermore, fiber structural parameter optimization will be conducted to reduce the fundamental mode loss, thus enhancing the HOMs extinction ratio and beam quality. By introducing beam quality improvement and maintaining its structural stability, the proposed ARF shows potential for applications in high-power and compact laser systems.