Laser-diode-pumped compact all-solid-state lasers with a high repetition frequency, short pulse width, and high beam quality have a wide application range in laser ranging, LIDAR detection, laser communication, and industrial processing. However, the thermal effect of solid-state lasers under high-power pumping is a crucial factor limiting the combination of the high single-pulse energy, high repetition frequency, and high beam quality characteristics of pulsed lasers. The burst-mode laser technology produces laser pulses in an intermittent mode, which alleviates the thermal effect of the continuous output of pulsed lasers to a certain extent. Common rod solid-state lasers are often end-pumped, which inevitably causes uneven heat distribution in the crystal. The slab laser reduces the temperature difference of the crystal by increasing the cooling area of the gain medium and makes the laser propagate along the zig-zag shape in the direction of the temperature gradient, which can reduce the thermal effect on the laser output to a certain extent. However, owing to the asymmetric shape of the slab gain medium, the output laser pulse is often elliptical or elongated in cross-section, resulting in a large difference in the beam quality in the x and y directions. In this study, beginning with the structure of the slab gain medium, we optimize the design of the gain medium size and aspect ratio and limit the range of the crystal doping region to reduce the thermal effect and optimize the laser spot. We hope that this method, which improves the beam quality of burst-mode lasers by optimizing the crystal structure of slabs, can also promote the development of high-energy, higher-repetition-frequency, and high-beam-quality burst-mode lasers.
In this study, Nd∶YVO4 crystals with large stimulated emission cross-sections and short upper-energy-level lifetimes are side-pumped by an laser diode (LD), and both sides of the doped region are bonded with non-doped YVO4 crystals to increase the heat dissipation area and constrain the pumping region, which facilitates heat dissipation and achieves high-repetition-frequency laser operation. By designing the crystal doping area aspect ratio to obtain a square beam exit surface, transmitting the laser pulse along the zig-zag shape in the doped region, and emitting the laser pulse perpendicular to the crystal end surface, the cavity mode optimization design is combined to limit and optimize the beam quality of the laser in the thickness and width directions to make both similar. In addition, the mode-selective effect of the small-aperture diaphragm is combined to further optimize the beam quality and obtain a pulse laser with the similar beam quality in the x and y directions. Electro-optical Q-switching technology is used to achieve a high repetition frequency and short pulse width output, and the transmittance of the output coupling mirror is optimized to obtain high-power burst-mode lasers.
Under the condition that the optimal transmittance of the output coupling mirror is 40% and the sub-pulse Q-switched repetition frequency is 80 kHz, a 1064-nm burst-mode laser with the highest average power of 5.03 W is obtained, with beam quality factor (M2) values of 2.67 and 2.43 in the x and y directions, respectively (Fig. 4). It can be seen that the beam qualities are similar in both directions, but there are certain higher-order-mode oscillations accompanied by stray light. By adding a small-aperture diaphragm with a diameter of 1 mm in the cavity to filter out stray light and suppress some of the higher-order-mode oscillations, a burst-mode laser with an average power of 2.56 W and a sub-pulse width of 7.2 ns is obtained (Fig. 5), and the M2 values in the x and y directions are 1.42 and 1.49, respectively (Fig. 6). This indicates that a high-energy, high-frequency, high-beam-quality burst-mode laser with similar beam quality in the x- and y-directions is realized in this study. In addition, this study measures the output waveforms of the burst-mode lasers at different sub-pulse repetition frequencies (Fig. 7) and the variation in sub-pulse width with Q-switched frequency at maximum pumping power (Fig. 8).
By optimizing the aspect ratio of the high-gain Nd∶YVO4 slab crystal to limit the generation of higher-order modes in the doping region and constraining the M2 values in the width and thickness directions to be approximately equal, a 1064-nm burst-mode laser with a high repetition frequency, short nanosecond pulse width, and high beam quality is obtained using LD-side quasi-continuous pumping and electro-optical Q-switching techniques. With the optimal transmittance of the output coupling mirror at 40%, the 1064-nm pulsed laser is obtained with an average power of 5.03 W and a sub-pulse Q-switched repetition frequency of 80 kHz, and the M2 values in the x and y directions are 2.67 and 2.43, respectively. By adding a small-aperture diaphragm in the cavity to filter out stray light and suppress the oscillation of some higher-order modes, a burst-mode laser with an average power of 2.56 W and a sub-pulse width of 7.2 ns is obtained. The M2 values in the x and y directions are 1.42 and 1.49, respectively. This approach provides a new research idea for the design of high-energy, high-frequency, and high-beam-quality burst-mode lasers.
