Compact and lightweight lidar systems have been investigated extensively for airborne and spaceborne applications. Portable and lightweight ultraviolet (UV) lasers are demanded in applications such as aircraft flight-safety warning and stratospheric detection. To achieve submillijoule-level single-frequency tunable UV lasers, the ramp-hold-fire-based seed-injection technique can be employed. However, this technique requires complex frequency stabilization schemes to align the modes within the main oscillator cavity with the seed laser frequency, which is not conducive to the compactness and miniaturization of the optical source structure. Additionally, conventional fiber-coupled pump laser sources are affected by issues such as large fiber bending radii and repeated coupling optical paths, thus resulting in technical redundancy and low space-utilization efficiency. In this study, we report an all-solid-state UV laser that uses direct end-pumping. Instead of adopting the fiber-homogenization role of conventional pump sources, we utilize a coupling optical path to directly shape the pump laser diode and introduce it into a Nd∶YVO₄ crystal. Additionally, we adopt master oscillator power amplifier (MOPA) technology, where we use a self-developed passively Q-switched non-planar ring oscillator (NPRO) as the seed light source to ensure the laser single-frequency characteristics. The pump laser employing this scheme occupies only 35.6% of the volume of a fiber-coupled pump laser, thus presenting a novel approach for achieving a more compact solid-state laser.

We first employ a fast-axis collimating lens to collimate the fast axis of a single-bar laser diode. Subsequently, a beam-rotating lens is utilized to rotate the originally uncollimated x-axis direction of the pump laser into the y-direction, which is easier to collimate. Subsequently, the y-direction of the laser diode is directly collimated using a cylindrical lens, whereas the x-direction is adjusted via a beam-reducing system comprising two cylindrical lenses. By adjusting the positions of these lenses, a superimposed pump beam is obtained, as shown in the simulation results presented in Fig. 3. The entire laser system adopts MOPA technology. The seed source is a passively Q-switched non-planar ring oscillator laser developed in our laboratory, which effectively ensures the single-frequency characteristic and compactness of the light source. The amplifier employs a double-pass amplification structure, where a directly end-pumped shaped laser diode is used as the pump source. Two LiB₃O₅ (LBO) crystals are used to achieve 355 nm light output. The schematic diagram of the laser structure is shown in Fig. 1.

By fine-tuning the spatial distribution of the pump light and its modal alignment with the signal light, the overall system can achieve optimal output performance. The pump laser, which undergoes output shaping, exhibits a Rayleigh length of 14 mm and a focal spot size characterized by D_x=1.00 mm and D_y=0.63 mm. By varying the injection current using this pump source, the amplification power as a function of the pump light peak power is obtained, as depicted in Fig. 5. The maximum optical-to-optical conversion efficiency is 14.7%. A comparison of the beam field distribution before and after the amplification of the fundamental light shows that using directly shaped pump lasers does not compromise the beam output quality. The beam quality factors after amplification are $M_x^{\mathrm{2}}=\mathrm{1.45}$ and $M_{\mathrm{y}}^{\mathrm{2}}=\mathrm{1.60}$, as shown in Fig. 6. Based on nonlinear frequency conversion, a 355 nm target light is obtained, and its optical characteristics are tested, as illustrated in Fig. 7. We present a schematic illustration of the designed laser mechanical structure, which features an estimated volume of 130 mm×100 mm×36 mm and a mass of 0.53 kg. Without considering fiber coiling and the fiber output coupling optics, the pump source volume in this shaping scheme accounts for only 35.6% of the volume of the fiber-coupled pump laser, thus significantly reducing the spatial footprint of the pump source. This presents a novel approach for achieving compactness in solid-state lasers.

This article reports a compact single-frequency UV all-solid-state laser designed for stratospheric-wind-detection lidars. The shaping of a single-bar laser diode is designed and implemented, which significantly improves the space utilization compared with fiber-coupled lasers. Through our experiments, we achieve an optical conversion efficiency of 14.7 % with a directly end-pumped Nd∶YVO₄ laser crystal, which is consistent with the pumping effect obtained using a fiber-coupled laser diode. At pump widths of 100 μs and 200 μs under a pump power of 60 W, a 40 μJ laser energy is amplified to 0.85 mJ and 0.925 mJ, respectively. Finally, we obtain a 0.225 mJ output from 355 nm light with an optical-to-optical conversion efficiency of 24.3%. The laser can realize higher pulse repetitions and stabilized laser for laser application systems.