Semiconductor lasers are highly efficiency and have a small divergence angle, narrow pulse width, and good durability. They are widely used in material processing, industrial manufacturing, laser lighting, lidar, and laser communication. However, the output spot of the traditional edge-emitting semiconductor laser is elliptical and an additional beam shaping system is required. Further, it is sensitive to temperature change because the temperature drift coefficient at the central wavelength is 0.3 nm/℃. Therefore, for a wide temperature range and large temperature differences between day and night, an additional temperature control system is required, which increases the volume, cost, and complexity of the laser. Recently, vertical-cavity surface-emitting lasers (VCSELs) have been used increasingly as semiconductor laser pump sources. Compared with traditional edge-emitting semiconductor lasers, VCSELs have circular output spots with a small divergence angle, good beam quality, small temperature drift coefficient, high reliability, and low cost. A VCSEL chip contains hundreds or thousands of units in the two-dimensional (2D) array distribution. Its high-power output of hundreds of watts or even kilowatts can be realized through the 2D array arrangement. Therefore, it is a suitable pump source for compact high-power solid-state lasers. In this paper, we report a laser with a VCSEL array as the pump source. The laser has the advantages of high beam quality, small volume, compact structure, and insensitivity to temperature change. It can be used under large day-to-night temperature differences as well as rapid temperature changes. Additionally, it serves as an emission light source for applications in space.

The laser uses an 808-nm VCSEL array as the pump source to pump Nd∶YAG crystal via end pumping. However, directly using it to pump the Nd∶YAG crystal will lead to low power density and low pump efficiency because the VCSEL array is composed of many units and has a large luminous area. Therefore, collimating and shaping the output laser of the VCSEL array is fundamental. To shape the output laser of each unit in the VCSEL array, we employ a 2D microlens array with the same distribution as that of VCSEL. The shaped VCSEL pump light is coupled to the crystal with a focusing lens. The doping concentration (atomic fraction) of Nd∶YAG crystal is 0.5%, and its size is 3 mm×3 mm×15 mm. The pumping side of the crystal is coated with 1064 nm high-reflection film and 808 nm high-transmission film. The output side is coated with 1064 nm and 808 nm high-transmission films. The electro-optic Q-switched module consists of a thin-film polarizer, KTP (KTiOPO₄) crystal, and quarter-wave plate. It adopts a voltage-increased electro-optic Q-switched method and is driven by a high-voltage signal to realize the on-and-off switch. A flat mirror with transmissivity of 40% is used for output coupling, which forms a laser resonator with the gain crystal pumping side to realize the normal operation of the laser. The total length of the laser is 120 mm.

At a repetition frequency of 100 Hz, when the VCSEL pump energy is 28.25 mJ (working current is 150 A), the static output energy and dynamic output energy are 7.96 mJ and 5.36 mJ, respectively. The ratio between dynamic output energy and static output energy for Q switching is 67.3%, and the optical-optical conversion efficiency is 18.9% (Fig. 6). Furthermore, the full width at half maximum of the output laser is 4.16 ns (Fig. 7) and the peak power is 1.29 MW. The beam quality factor along the two directions is Mx2=1.409 and My2=1.531 (Fig. 9). In the temperature range of 10-37 ℃, the minimum and maximum output energies are 5.24 mJ and 5.58 mJ, respectively.

In this paper, we propose a compact solid-state laser with a small volume and large operating temperature range. The pump module consists of VCSEL and microlens arrays, and the VCSEL output laser is shaped using the microlens array. The Nd∶YAG crystal is pumped via end-pumping. Through the electro-optic Q-switching method, the laser is obtained with a pulse width of 4.16 ns, output energy of 5.36 mJ, and beam quality factors of Mx2=1.409 and My2=1.531. With the advantages of good beam quality, small volume, compact structure, and insensitivity to temperature change, the laser proves efficient under large day-to-night temperature differences and rapid temperature changes and can further be used as an emission laser source for application in space.