Cylindrical vector beams (CVBs) have attracted much attention due to their unique doughnut intensity profiles and axisymmetric polarization state distribution^[1]. They show significantly different tight focusing characteristics from the traditional linearly, circularly, and partially polarized light when using a high numerical aperture microscope objective^[2]. Therefore, CVBs hold great potential for applications in super resolution imaging^[3], surface plasma resonance^[4], optical tweezers^[5], electron acceleration^[6], and laser processing^[7]. In particular, in fields such as super-resolution imaging and high-capacity optical fiber communication systems, there is growing interest in utilizing flexible optical fibers to output CVBs.

In addition to the fundamental mode LP01 (HE11x/y), the few-mode fiber (FMF) can support the second-order mode LP11. The LP11 mode comprises four degenerate vector modes: TE01, HE21e/o, and TM01, where TE01 and TM01 represent azimuthally and radially polarized modes, respectively. Therefore, the FMF is one of the ideal tools for producing CVBs in fibers. The key to producing high-purity CVBs lies in the design of the efficient mode coupler and mode selector using an FMF^[8–12]. The larger the effective refractive index difference between the second-order degenerate modes in the FMF, the weaker the coupling between the modes, and the more stable they become. Typically, a step index FMF consisting of a double layer structure with a circular core and ring cladding is utilized^[13–16]. This FMF is capable of achieving the separation of second-order degenerate modes, so it has been intensively used to generate the CVBs. However, the effective refractive index difference between the degenerate modes is only 10−5^[17].

As a special structure of the FMF, ring core fibers (RCFs) are designed for spatial division multiplexing systems due to their better transmission of high-order modes^[18]. They feature a circular inner cladding, a ring core, and a ring outer cladding^[19]. These fibers can generate the second-order modes easily, and the effective refractive index difference between the second-order degenerate modes can reach 10−4^[17,20], which is at least one order of magnitude higher than traditional step index FMFs. This makes it easier to separate the degenerate modes^[21]. Currently, most research on RCFs focuses on their structural design and preparation^[22] as well as the generation and transmission of vortex beams and orbital angular momentum modes^[23–26] in these fibers. There have been few studies investigating the preparation of fiber Bragg gratings (FBGs) in RCFs and their applications in generating CVBs. The theoretical analysis results show that azimuthally polarized light with high mode purity^[27] and vector mode conversion using the tilted FBG^[28] in an RCF can be realized. A FBG pair in a step index FMF was used to form a laser resonator and to realize a CVB laser with low threshold and high conversion efficiency with the help of a ring-core ytterbium-doped fiber^[29]. Subsequently, a layer of aluminum metal on the surface of a corroded FMF was deposited to suppress other modes, except angular polarization, thus realizing azimuthally polarized CVB laser output^[17], but the ring-core ytterbium-doped fibers were only used as the gain medium. A single-mode fiber (SMF)/RCF fused tapered coupler was used to achieve a CVB laser by coupling the fundamental mode to the second-order mode^[30]. Our team used an acousto-optic mode coupler to generate the CVBs and vortex beams in the RCF^[31], but further improvements were needed due to the complex system structure and purity issues regarding mode quality.

In this Letter, we propose a new method for generating an all-fiber CVB based on an FBG inscribed in an RCF by the femtosecond laser phase mask scanning technique. This method involves a linear fiber resonator consisting of a high reflectivity FBG in an SMF and a low reflectivity FBG in an RCF, which serves as both mode selector and laser output reflector. By adjusting the polarization controllers, switchable LP01 and LP11 mode lasers can be achieved. Meanwhile, azimuthally and radially polarized CVBs can be successfully generated.

A germanium-doped RCF is utilized to induce high-order mode oscillation. Its cross-section micrograph captured by a microscope (Zeiss Axio Scope. A1) is shown in Fig. 1. The RCF comprises an inner cladding, a ring core, a trench, and an outer cladding, with corresponding refractive indices of 1.444, 1.466, 1.437, and 1.444, respectively. The diameters of the inner core, outer core, and cladding are 14.2, 24.2, and 125 µm, respectively. The low refractive index trench effectively suppresses the crosstalk between different modes. COMSOL Multiphysics software is employed to simulate the mode field distributions based on the structure parameters and refractive indices provided above. The propagation modes in this RCF and their mode field distributions are depicted in Fig. 2. It can be seen that this fiber supports two scalar modes: LP01 and LP11. The LP01 comprises two degenerate fundamental vector modes, HE11x/y, while the LP11 comprises four degenerate second-order vector modes, TE01, TM01, and HE21e/o. The black arrow indicates the direction of the electric field vector.

The relationships between the effective refractive indices of the first- and second-order modes LP01 and LP11 in the RCF and the incident laser wavelength are illustrated in Fig. 3. We can see that the effective refractive index of each mode in the RCF increases linearly with a decrease in the incident laser wavelength. Specifically, at the wavelength of 1550 nm, the effective refractive index difference between the TM01 mode and the TE01 mode is 2×10−4, while it is 1×10−4 between the HE21e/o mode and the TM01 or the TE01 mode. This difference is significantly larger than that of ordinary step-index FMFs. So the separation of vector modes is easily achieved in the RCF, leading to a strong suppression effect on the crosstalk between different modes within this fiber. As a result, long-distance stable transmission of the desired vector modes becomes readily achievable.

The experimental setup of the all-fiber CVB laser is illustrated in Fig. 4. The single-mode erbium-doped fiber (EDF, OFS MP980) with a length of approximately 4.5 m is pumped by a 974 nm laser diode (LD) with a maximum output power of 800 mW through a 980/1550 nm wavelength division multiplexer (WDM). A single-mode FBG (SMFBG) with a reflectivity of 99% around 1543.32 nm and a ring core FBG (RCFBG) with a reflectivity of about 20% serve as the high and low reflectors of the linear fiber resonator, respectively. Polarization controller 1 (PC1) is utilized to remove the degeneracy of the second-order modes, while polarization controller 2 (PC2) is positioned on the RCF to adjust the polarization of the output CVB laser. A 30 cm section of the RCF was fusion spliced with an SMF with a core offset. The lateral offset splicing spot (OSS) facilitates mode coupling from the fundamental mode of the SMF to the second-order modes of the RCF^[32]. The output power and laser spectrum were measured by a power meter and an optical spectrum analyzer (OSA, Yokogawa, AQ6319), respectively. An optical fiber collimator (C), a polarizer (P), and a near-infrared CCD were used to measure the intensity distribution and the polarization of the CVB laser.

Although a traditional UV laser can also be used to inscribe the FBGs, considering its low photosensitivity and large core diameter of the RCF, the femtosecond laser and phase mask scanning method were adopted to inscribe the RCFBGs. After stripping its coating, the RCFs were illuminated using 35 fs laser pulses from a Ti: sapphire amplifier laser (Spectra-physics SPFIRE ACE-35F-1KXP) with a central wavelength of 800 nm and a repetition rate of 1 kHz. The attenuated femtosecond laser was focused onto the ring core using a cylindrical lens through a silica zero-order nulled phase mask (Ibsen Photonics). The phase mask is designed for 800 nm illumination with a period of 2135 nm. At the same time, the cylindrical lens and the phase mask were mounted on a 3-axis piezo stage (Thorlabs MAX302/M) to scan the RCF vertically with an amplitude of 20 µm and a frequency of 0.1 Hz. The reflection spectra of the RCFBGs were monitored using an amplified spontaneous emission source (Exfo, FLS-2300B) and an OSA. A dark-field microscopic imaging system was utilized to monitor the ring core in real time and adjust the laser focus line on the ring core accurately^[33], thanks to the weak nonlinear photoluminescence radiation from the core region, which can be excited by a femtosecond laser in the germanium-doped RCF.

The reflection spectra of the RCFBG and the SMFBG are depicted in Fig. 5. We can see that the reflection spectrum of the RCFBG exhibits three peaks, with wavelengths of 1543.34 and 1535.75 nm corresponding to the self-coupling of the LP01 and LP11 modes, respectively, while the wavelength of 1539.52 nm corresponds to the cross-coupling between LP01 and LP11 modes. This indicates that the wavelength spacing between the LP01 and LP11 modes is 7.59 nm. When the central wavelength corresponding to the LP01 mode of the RCFBG coincides with that of the SMFBG, the CVB generated from the OSS will be transmitted through the RCFBG. Meanwhile, most of the fundamental mode will be reflected back into the resonator, thereby creating a laser oscillating in fundamental mode within the resonator^[34]. The presence of the fundamental mode is a critical factor influencing both the output mode and the polarization purity.

In order to investigate the impact of ring core offset fusion splicing on the laser output characteristics, the offsets between the SMF and the RCF were incrementally increased from 0 to 12 µm by manually adjusting the drive motor of a fusion splicer (FSM-60 S), as shown in Figs. 6(a1)–6(a5). The real-time intensity distribution of the transmission modes in RCF was monitored using an infrared CCD, as shown in Figs. 6(b1)–6(b5). We can see that as the offset increases, there is a gradual transition from the fundamental mode to a second-order mode. Once the offset reaches a certain value, there is a sharp decrease in intensity within the fiber core, as most of it is coupled to the cladding and cannot be effectively transmitted through the core anymore. The inner core and outer core radii of the RCF are 7.1 and 12.1 µm, respectively. The mode field energy is predominantly concentrated in the center of the ring core. Therefore, the optimal offset is approximately 10 µm. This conclusion is further confirmed by the experimental results presented in Fig. 6.

The broadband light in the range of 1525 to 1565 nm is emitted when the pumped light is coupled into the resonator and absorbed by the EDF. The light at a wavelength of 1543.32 nm will be reflected back and forth by the SMFBG and the RCFBG, resulting in continuous amplification. When it reached the OSS, part of the fundamental mode LP01 will be converted into the second-order mode LP11. By controlling the losses in the resonator by adjusting the polarization controllers, only the fundamental mode oscillates in the resonator while the second-order modes can selectively pass through the RCFBG^[33], and the switchable LP01 and LP11 mode lasers can be achieved. Figures 7(a) and 7(b) illustrate the laser output spectra for the LP01 and LP11 modes at a pump power of 508 mW, respectively. The oscillation laser of the LP01 mode is located at 1543.32 nm with a 3 dB bandwidth of 0.07 nm, and the signal-to-noise ratio exceeds 43 dB. Moreover, the oscillation laser of the LP11 mode is located at 1535.73 nm with a 3 dB bandwidth of 0.18 nm, and the signal-to-noise ratio exceeds 38 dB. These results suggest that this laser exhibits a narrow bandwidth and excellent monochromaticity.

The intensity distributions of the LP11 mode laser measured when the core offset is increased from 9 to 10.5 µm are shown in Fig. 8. It can be observed that the intensity and mode purity of the output laser are relatively good when the offsets are 9.5 and 10 µm. This is because the radius of the ring core is approximately 9.6 µm, and when the fiber core of the SMF aligns with either side of the RCF, the highest coupling efficiency between the two fibers is achieved. The micrograph of the OSS between the SMF and the RCF is given by the inset in Fig. 4. By reducing the pre-melting power and discharge power, while extending the discharge time and re-discharge time, we can prevent bending of the RCF at the fusion point, thus minimizing the light loss during the transmission between the SMF and the RCF.

High-purity CVBs are achieved through precise adjustment of the two polarization controllers. PC1 is utilized to ensure that the fundamental mode oscillates in the resonator at a central wavelength of 1543.32 nm and to guarantee that the output beam from the RCFBG is in the LP11 mode. Meanwhile, PC2 is fine-tuned to suppress the degeneracy of the LP11 mode, resulting in an output laser that is azimuthally or radially polarized. The CVBs display a distinctive doughnut-shaped intensity pattern. The beam intensity profiles of the TE01 and TM01 modes are presented in Figs. 9(a1) and 9(b1), respectively. The polarization characteristics of the output CVB are measured using a linear polarizer with an adjustable polarization direction. Figures 9(a2)–9(a5) illustrate the intensity distributions of the TE01 mode after passing through a polarizer set at angles of 45°, 90°, 135°, and 180°, respectively. By carefully adjusting the polarization controllers, we can also obtain a TM01 mode shown in Figs. 9(b2)–9(b5), indicating switchable transverse modes through the additional polarization controllers on both sides of the OSS within the resonator.

The pump power was gradually increased to characterize the mode conversion efficiency of the laser. The relationships between the pump power and output power are shown in Figs. 10(a) and 10(b) for the TE01 and TM01 modes, respectively. The threshold power, at which the laser starts to oscillate, is approximately 7.6 mW. Below this power, the negligible signal light can be detected, while above the threshold, the output power increases almost linearly with the pump power. The TE01 mode has a mode conversion efficiency of 0.61%, and it is 1.07% for the TM01 mode. The difference between the two modes is due to the fact that the polarization state in the laser resonator is controlled by adjusting the polarization controller, causing different mode losses. In addition, the severe mode mismatch between the SMF and the RCF and large light coupling losses at the OSS result in lower mode conversion efficiency of the laser. It may be improved by optimizing the fusion splicing between the SMF and the RCF.

The mode purity of the TM01 mode was evaluated using a one-dimensional intensity distribution estimation method, with its optical field intensity given in Fig. 11. It can be clearly seen that the optical field of the TM01 mode forms a hollow ring pattern. There remains some residual energy at the center of the optical field, resulting from incomplete conversion of the fundamental mode. The intensity of the two peaks indicates the extent to which the fundamental mode has been converted to second-order vector modes, with a ratio of 1:9. This suggests that the mode purity of the TM01 mode is approximately 90%.

We present a method for generating CVBs using an RCFBG. The all-fiber resonator consists of a high reflectivity SMFBG and a low reflectivity RCFBG, which serve as the mode selector and laser output reflector, respectively. Due to its large diameter of the ring core, the femtosecond laser phase mask scanning technique is used for RCFBG inscription. There are three peaks in the reflection spectrum of the RCFBG corresponding to the self-coupling of LP01 and LP11 modes, as well as cross-coupling between the LP01 and LP11 modes. Additionally, a lateral offset splicing spot is introduced to facilitate coupling of the fundamental mode to the second-order modes due to its unique ring core structure of the RCF. By adjusting the polarization controller, switchable LP01 and LP11 mode lasers can be achieved. Meanwhile, azimuthally and radially polarized CVBs are successfully realized. The mode purity of the TM01 mode is about 90%. Although the mode conversion efficiencies for azimuthally and radially polarized beams are relatively low, they may be improved by optimizing the fusion splicing between the SMF and the RCF. Overall, our findings demonstrate an effective method for generating CVBs using an RCFBG within an all-fiber resonator.