Youqiang Liu

To generate cylindrical vector beams (CVBs) with high mode purity and high beam quality, a polarization-maintaining fiber oscillator was built, followed by high-efficiency mode conversion with an S-waveplate. This system leverages the S-waveplate as a mode converter to transform the fundamental LP₀₁ mode into radially and azimuthally polarized beams, which both feature a central polarization singularity. The experimental results demonstrate that the system successfully yielded a radially polarized beam with an output power of 3.93 W and a high mode purity of 96.99%, as well as an azimuthally polarized beam with an output power of 3.71 W and a mode purity of 96.5%. The corresponding beam quality factors (M²) were measured to be 2.17 and 2.01, respectively. After 60 min of continuous operation at maximum power, the radially and azimuthally polarized beams had average powers of 3.988 W and 3.785 W, with RMS stabilities of 0.788% and 0.590%, respectively. To the best of our knowledge, this represents the highest power reported to date for both radially and azimuthally polarized CVBs that simultaneously possess high mode purity and beam quality. This work provides an effective solution to obtain high-power CVB sources that simultaneously possess high mode purity and beam quality, establishing a solid foundation for subsequent power scaling and practical applications.

In this manuscript, we demonstrated a switchable single-/dual-wavelength Yb:YAG laser based on intracavity loss modulation in a dual-confocal resonator configuration. The laser employed a dual-confocal resonator with a length of 175 mm that enabled the controlled adjustment of geometric losses through precise angular modulation of the output coupler, achieving stable switching between single- and dual-wavelength operations. Under a maximum pump power of 20.98 W, the laser delivered 1.868 W of single-wavelength output at 1050 nm with an optical-to-optical (O–O) conversion efficiency of 8.9%, exhibiting excellent power stability (RMS < 0.6%). When it was switched to a dual-wavelength operation at 1030 nm and 1050 nm, the total output power reached 0.376 W with respective powers of 0.180 W (1030 nm) and 0.196 W (1050 nm), corresponding to a 1.792% O–O conversion efficiency and a power stability of RMS < 3.0%. When it was switched to single-wavelength operation at 1030 nm, the maximum output power reached 0.193 W with a 0.920% O–O conversion efficiency and a power stability of RMS < 1.5%. These experimental results confirm that the controlled modulation of geometric losses in a dual-confocal resonator provides an effective approach for achieving flexible wavelength switching while maintaining stable output performance. This methodology offers promising potential for expanding the application scope of dual-wavelength lasers in precision optical systems.

CVBs (cylindrical vector beams) are widely used in optical imaging, optical trapping, material processing, etc. In this study, based on mode-selective couplers and passive mode-locking fiber technology, a cylindrical vector fiber amplifier with an MOPA (master oscillator power amplifier) structure was developed. In the experiment, the pre-amp stage reached 19.87 mW output power and a CVB output with a mode purity greater than 97%. The measured beam quality factor was M² = 2.1. The CVB output power obtained by the first-amp stage was 152.4 mW, and the mode purity was greater than 92%. The measured beam quality factor was M² = 1.99. The internal inhomogeneity and external effects of the isotropic LMA (large-mode-area) fiber led to a decrease in beam quality and mode purity. After amplification, the gain of the fundamental mode was higher and the power was greater, resulting in a greatly reduced mode purity. This CVB fiber amplifier yielded important research value in expanding the applications of high-power fiber lasers.

In response to the demand for high-power, long-pulse-width 532 nm lasers in the medical and industrial processing fields, this paper explains how the laser cavity of a high-power Nd:YAG 532 nm laser can be extended while maintaining the laser’s q-parameter by using a 4f optical system. The results show that at a repetition rate of 10 kHz, the extended cavity achieved a maximum average power of 112 W. Compared with the short cavity, the power was not significantly reduced. The pulse width was extended from 56 ns to 85 ns, and its broadening ratio reached 46.5%. The laser maintained good beam quality during high-power operation.

Pseudo-Random Binary Sequence (PRBS) phase modulation is an effective method for suppressing the stimulated Brillouin scattering (SBS) effect generated by narrow-linewidth fiber lasers during amplification. We noticed that backward time-domain pulses were generated when using PRBS modulation signals in fiber amplification. In this paper, the time-domain dynamic characteristics of the forward output laser and the backward Stokes light after PRBS phase modulation were studied theoretically. Through analyzing the transient SBS three-wave coupling theory and combining it with the SBS accumulation time constant, we knew that the forward and backward high-intensity pulses were caused by the long dwell time of the PRBS. For this purpose, we provided a new method for suppressing high-intensity pulses caused by a long dwell time; namely, we modified the maximum length sequence (MLS) of PRBS signals to eliminate the long dwell time, took the PRBS-9 signal at 1 GHz as an example, and then used MLS1 modulation and MLS2 modulation to compare them with unoptimized PRBS modulation. The output laser peaks of the MLS1 and MLS2 signals were reduced from ±55% to ±25% and ±10% relative to the original PRBSs, respectively, and the peaks of Stokes light were reduced from 39% to 19% and 11%, respectively. Additionally, we experimentally verified that the rational optimization of the sequence did not reduce the SBS threshold. The results provided a new method for suppressing high-intensity pulses during the amplification of a PRBS phase-modulated laser, which played an important role in the output stability of high-power narrow-linewidth fiber amplifiers.

Cylindrical vector beams (CVBs) are the product of polarization modulation of optical fields, and possess both unique focusing characteristics and excellent properties applicable to machining, imaging, communication and other fields. Mode selection couplers comprise a promising new method to realize the long-term stable output of cylindrical vector beam all-fiber lasers. Mode selection couplers have the advantages of a simple structure, high mode conversion efficiency and high mode purity. However, the production process of conventional asymmetric mode selection couplers is more complicated. Therefore, in this paper, a symmetric two-mode coupler with a 1 μm band is designed and fabricated using the finite element method, beam propagation method and fused pull-cone method, and a set of all-fiber passive mode-locked lasers based on symmetric dual-mode couplers are constructed. Finally, we obtain cylindrical vector beam outputs with central wavelengths of 1038.97 nm/1067.72 nm, a repetition rate of 8.78 MHz, pulse durations of 660 ps/656 ps, maximum average powers of 5.25 mW/5.2 mW, and the high mode purity of TM (transverse magnetic)₀₁ mode and TE (transverse electric)₀₁ mode is 97.18% and 97.07%, respectively.

We developed a high-power, double-intracavity-frequency, diode-side-pumped, Q-switched Nd:YAG laser. The thermal birefringence effect of Nd:YAG crystal was compensated by a quartz rotating mirror, and the stable cavity structure was designed according to the thermal focal length of a single module. A 532 nm laser with a maximum linearly polarized output power of 96 W was obtained at a pumping power of 1300 W and repetition rate of 10 kHz. The pulse width was 55.8 ns, and the beam quality was measured to be M²_x = 5.98, M²_y = 8.14 at the maximum output power; therefore, a brightness as high as 6.97 × 10⁸ W·cm⁻²·Sr was achieved. To the best of our knowledge, this is the highest brightness obtained for a high-power Nd:YAG laser at 532 nm.

A diode-side-pumped, high-energy, high-beam-quality, pulsed solid-state Nd³⁺:Y₃Al₅O₁₂ (Nd:YAG) laser with a thermoelectric cooler (TEC) is investigated in this study. The pump laser was a pulsed laser diode array with maximum peak power of 15 kW. A 350 mJ laser pulse was obtained with a wavelength of 1064 nm, a pulse duration of 10 ns, a total electrical-to-optical efficiency of 7.5%, a relative stability of output energy of 5%, and a beam quality of M² < 4.