A laser beam with axially symmetric polarization, i.e., radial or azimuthal polarization, has attracted considerable interest due to its unique properties, including a donut-shaped intensity profile, uniform absorption in a material, generation of a longitudinal electric field component under tight focusing, and mitigation of thermally induced aberrations [1–3]. It is often referred to as a cylindrical vector (CV) beam and has been used in numerous applications such as optical trapping and manipulation of particles, particle acceleration, laser material processing, high-precision microscopy, and optical lithography [4–7].

A variety of methods have been developed to generate CV beams. The most straightforward approach is to convert a linearly (or circularly) polarized beam to a CV beam using special optical components such as a birefringent material [8], a q-plate [9], a sub-wavelength nanograting [10], a meta-surface device [11], a holographic liquid-crystal polarization converter [12], or an interferometric device [13]. These methods are simple and easy to implement, but suffer from a low purity of the polarization state. Alternatively, a CV beam can be generated directly from a laser resonator incorporating specially fabricated intracavity optical components, such as a q-plate [14], a conical Brewster prism [15], a multilayer polarization grating mirror [16], or thermally induced birefringent materials [17, 18]. Direct generation allows a beam to have a polarization state with a high degree of purity, but laser operation is sensitive to cavity conditions and laser alignment. Among these components for CV beam generation, the S-waveplate is a promising candidate due to its ease of use, flexibility, and high laser damage threshold [19–22]. It is a sub-wavelength nanograting device written in a fused silica substrate with grating directions that are dependent on the azimuthal angle, so it can convert a linearly polarized beam to a CV beam simply by passing a Gaussian beam through it. This can also be used as a polarization selector when it is placed in a laser cavity. Lin et al. [21] reported a cladding-pumped Yb fiber laser with >30 W of radially polarized output using an intracavity S-waveplate. Although it could successfully generate a high output power, the multi-mode fiber laser configuration used, comprising a free-space external feedback cavity with the S-waveplate, makes it difficult to excite and maintain the four high-order vector modes in the LP11 mode group of the fiber waveguide, so the laser system is very sensitive to operating conditions and environment including bending, temperature, pressure, and so on. Our group previously reported the direct generation of a CV beam from an Nd:YVO₄ laser incorporating an intracavity S-waveplate [22]. However, the output power was limited to less than 1 W due to the thermal lensing effect of Nd:YVO₄, exacerbated by the insertion loss of the S-waveplate. Therefore, it is expected that output power scaling can be achieved by employing a laser configuration with minimized thermal lensing, such as a thin-disc laser configuration.

A thin-disc laser uses a thin, disc-shaped active gain medium with a typical thickness of 100 μm to 250 μm, which is cooled by one of the flat faces of the disc [23–25]. Due to its excellent heat dissipation capacity, a thin-disc laser can produce up to several kilowatts of average output power while minimizing detrimental thermal effects including thermal lensing. A significantly higher output power of up to 750 W has been reported in a thin-disc laser with radial polarization [26]. They used a nano-written circular grating waveguide output coupler to force the polarization state so that its output polarization was fixed and could not be switched to the other polarization state, i.e., the azimuthal polarization state. Thus, we aim to build a thin-disc laser with switchable polarization, either radial or azimuthal polarization.

Here, we report on an Yb:YAG thin-disc laser with radially or azimuthally polarized outputs at 1,030 nm incorporating an intracavity S-waveplate. We could easily select either radial or azimuthal polarization states by adjusting the rotation angle of the S-waveplate, producing >10 W of CW output for both polarization states.

The experimental set-up for the Yb:YAG thin-disc laser is schematically shown in Fig. 1(a). We used a 7.2 at.% Yb:YAG thin-disc with a thickness of 215 μm and a diameter of 12 mm as the gain medium. The front face of the disc was anti-reflection (AR) coated and the rear face was high-reflection (HR) coated at both pump and laser wavelengths. The disc was bonded to a water-cooled CuW heat sink maintained at 15 ℃. Pump light was provided by a fibre-coupled high-power laser diode stack wavelength-locked at 969 nm. The pump beam was imaged onto the thin-disc with a beam size of 4.0 mm, using a collimation optic and a parabolic mirror after a beam homogenizer. The unabsorbed pump beam reflected from the rear face of the disc was collimated again by the parabolic mirror and redirected to the disc using two mirrors and another part of the parabolic mirror while having the same beam size as the incident beam on the disc. With these conditions, the pump beam passed through the Yb:YAG disc 24 times, resulting in a pump absorption efficiency of ~95%.

Figure 1. Experimental setup and conditions. (a) Experimental setup for the thin-disc laser, (b) pump beam profile and fluorescence from the Yb:YAG thin-disc measured by CCD camera (inset), and (c) TEM₀₀ mode size as a function of the position from the HR mirror. HR mirror, High reflectance mirror; TD, thin-disc module; CC, concave mirror; CX, convex mirror; OC, output coupler; SW, S-waveplate; TFP, thin film polarizer; CCD, charge-coupled device.

Figure 1(b) shows the pump beam profile and its fluorescence (the inset) measured from the Yb:YAG thin-disc under pumping, confirming a homogeneous pump beam profile. The laser resonator comprised an HR mirror providing feedback for lasing at one end of the resonator, a thin-film polarizer, a concave mirror (CC) with a radius of curvature (ROC) of 3,000 mm, a convex mirror (CX) with a ROC of 1,000 mm, two HR mirrors for folding the beam path, and a planar output coupler with 93% reflectance at the lasing wavelength. The lengths of the arms in Fig. 1(a) were ~300 mm, ~400 mm, ~400 mm, and ~400 mm for the HR-TD, TD-CC, CC-CX, and CX-OC, respectively. The total length of the resonator was ~1,500 mm, resulting in a calculated TEM₀₀ mode diameter of ~2.9 mm at the disc. The mode size as a function of the position from the HR mirror is shown in Fig. 1(c). With these conditions, the Yb:YAG thin-disc laser yielded 157 W of CW output for an incident pump power of 357 W, corresponding to a slope efficiency of 54.2%. The output was linearly polarized, and its polarization extinction ratio was measured to be ~30 dB. The output power as a function of the pump power is shown in Fig. 2. The inset in Fig. 2 also shows the transverse output beam profile, and the measured beam qualities were 1.1 (x-axis) and 1.2 (y-axis) at the maximum output power, confirming that the output was a nearly diffraction-limited TEM₀₀ mode.

Figure 2. Yb:YAG thin-disc laser output power with linear polarization as a function of the incident pump power. The inset is the measured transverse beam profile.

To generate laser output with radial or azimuthal polarization, the S-waveplate was inserted in the cavity, as shown in Fig. 1(a). In [22], our group demonstrated, both theoretically and experimentally, that the S-waveplate in the laser cavity allows the laser output to have a radial or azimuthal polarization state. The S-waveplate used in our experiment had a clear aperture of 4 mm and was fabricated for light in a wavelength regime of 1,030 ± 20 nm. As the S-waveplate is recommended for a collimated beam that is >1 mm in diameter, it was placed in the vicinity of the planar output coupler, which had a calculated TEM₀₀ mode size of ~1.1 mm. The polarization state (radial or azimuthal polarization state) could be selected by rotating the S-waveplate.

With these conditions, the Yb:YAG thin-disc laser yielded 10.8 W and 10.2 W of CW outputs with radial and azimuthal polarization, respectively, for an incident pump power of 131 W, as shown in Fig. 3. The slope efficiencies were calculated to be 22.9% and 23.7% for the outputs with radial and azimuthal polarization, respectively. The slight difference in laser outputs between the two polarization states can be attributed to the alignment sensitivity when rotating the S-waveplate. At higher pump power levels, it was difficult to maintain the radial or azimuthal polarization state due to thermal lensing, which was exacerbated by the low laser efficiency. Therefore, we limited the pump power to 131 W, although the output in Fig. 3 did not show signs of rollover. Figure 4 shows the output beam profiles of the Yb:YAG thin-disc laser and its polarization states. Both radially and azimuthally polarized beams had donut-shaped intensity distributions (the insets), which are in good agreement with the theoretical curve of the first-order Laguerre-Gaussian (LG₀₁) mode. The beam qualities (M²) were also measured to be ~2.2 (x-axis) and ~2.3 (y-axis) for both polarization states, proving that the outputs were the LG₀₁ mode [27]. The polarization state was confirmed by the beam profiles after passing the beam through a cube polarizer, shown in Fig. 4 that the laser output had radial or azimuthal polarization states. The purities of the polarization states were calculated to be 92.1% and 96.4% for the radial and azimuthal polarization states, respectively, confirming that the laser output had the high-purity polarization state as expected.

Figure 3. Yb:YAG thin-disc laser output power with radial or azimuthal polarization as a function of the incident pump power.

Figure 4. Calculated (solid lines) and measured (symbols) transverse intensity profiles of the laser outputs with (a) radial, or (b) azimuthal polarization measured at each maximum output power. The inset pictures in the graphs are the transverse intensity profiles of the outputs and the bottom pictures show the transmitted beam profile after the cube polarizer. The arrows indicate the direction of the polarizer. The beam profiles were monitored by silicon CCD camera (SP620U; Ophir-Spiricon LLC., Jerusalem, Israel).

Although we successfully generated laser outputs with radial or azimuthal polarization with powers ~17 times higher than that of the Nd:YVO₄ laser in a previous experiment [22], the laser efficiency was still much lower than expected, limiting the maximum achievable output power. We first assumed that this low slope efficiency was caused by a high round-trip loss due to the intracavity S-waveplate. We roughly calculated the round-trip loss due to the S-waveplate from the following equation for the slope efficiency η as [28]

where T is the transmittance of the output coupler, L is the round-trip loss, η_Q is the quantum efficiency, η_o is the pump-TEM₀₀ mode overlap efficiency, and η_abs is the absorption efficiency. Using T = 0.07, η_Q = 0.94, η_o = 0.725, and η_abs = 0.95, the additional round-trip loss due to the S-waveplate was roughly calculated to be ~11% based on Eq. (1) using slope efficiencies of 54.2% and 23% for the lasers with and without the S-waveplate, respectively, if we attribute the low laser efficiency to the insertion loss of the S-waveplate. This loss was too high considering that the transmission of the AR-coated S-waveplate at ~1 μm was measured to be over 99%, so we believe that there should be other factors for the low laser efficiency. We attribute one of the other dominant factors to the mode mismatch between the TEM₀₀ and LG₀₁ modes in the resonator, since the S-waveplate was designed for polarization conversion rather than mode transformation. We measured the transformed LG₀₁ mode beam size after the S-waveplate and found that it was nearly the same as that of the incident TEM₀₀ mode beam. Since the resonating LG mode size in the cavity should be $\sqrt{2}$ times larger than the TEM₀₀ mode at the waist position [29], the LG₀₁ mode size transformed by the S-waveplate was smaller than the beam size required for resonating in the cavity, resulting in a mode mismatch and hence lower laser efficiency. By simple calculation [28, 29], the overlap efficiency between the ideal LG₀₁ mode and the transformed LG₀₁ mode was 0.79. Therefore, we expect a more efficient laser operation by forcing a transformed LG₀₁ mode beam to satisfy the resonance condition in the cavity along with a reduced round-trip loss, which is an ongoing work.

We have successfully demonstrated the high-power operation of an Yb:YAG thin-disc laser with radial or azimuthal polarized output incorporating an intracavity S-waveplate. The Yb:YAG thin-disc laser yielded 10.8 W of radially polarized output and 10.2 W of azimuthally polarized output at 1,030 nm for an incident pump power of 131 W at 969 nm. The maximum output was limited by low laser efficiency due to insertion loss and mode mismatching due to the S-waveplate in the cavity. Therefore, the use of a mode-transformation-optimized S-waveplate in combination with a careful cavity design should lead to an improvement in laser efficiency, opening up the prospect of radially or azimuthally polarized Yb:YAG thin-disc laser output power on the level of tens or hundreds of watts.

National Research Foundation of Korea (NRF) (Grant no. 2021R1A2C1007277); European Union and the state budget of the Czech Republic under the project LasApp (Grant no. CZ.02.01.01/00/22_008/0004573).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.