Wavelength spectra in the near-/mid-infrared range, especially near 2 μm, cover the absorption lines of various atmosphere gases—including the main greenhouse gases (GHGs) like CO₂ and_{CH4. As a result, high-energy lasers emitting at these wavelengths have garnered significant attention as light sources for light detection and ranging (Lidar) systems, especially in applications such as GHG detection. However, it remains a challenging task to develop lasers with compact size, high energy, high efficiency, and high robustness, which are demanded by airborne and spaceborne Lidars. Master oscillator power amplifier (MOPA) based on Tm, Ho co-doped materials is one of the most promising laser concepts. Owing to the absorption of Tm, these co-doped materials can be directly pumped by commercial laser diodes emitting at 793 nm and the high reabsorption loss can also be avoided by lowering the Ho doping concentration. Among the host materials, the birefringent crystal LiYF4 (YLF) stands out in high-energy Q-switched laser development owing to its low phonon energy and long upper laser level lifetime when doped with Tm and Ho. Thus, in this paper, we present a side-pumped high-energy MOPA laser amplifier designed for Lidar applications. The amplifier is based on a (Tm,Ho) ∶YLF crystal and operates at 2051 nm with a pulse width in the hundred-nanosecond range.}

In the MOPA system, we employ laser diode (LD) bars as pump sources, which emit light at 793 nm and have a maximum peak pump power of 100 W for each bar, with a pump duration of 1 ms. The active media used are homemade (Tm,Ho) ∶YLF rods doped with Tm (atomic fraction of 5%) and Ho (atomic fraction of 0.5%). Both the LD bars and laser crystal rod are assembled in a self-designed triple side-pumped laser head module, together with conductive-cooling heat sinks and a wedged lens pump light coupling system. In the master oscillator (MO), an 8-shaped ring cavity with four mirrors and a length of 1.7 m is constructed to achieve pulses with a hundred-nanosecond duration, and Q-switching is realized using an acousto-optic modulators (AOM). In the power amplifier (PA), a 3-stage double pass amplification is employed; thus, the maximum required pump energy/peak power in each stage can be reduced. In addition, the laser beam quality is optimized by mounting the crystal rod of secondary amplifier with an orthogonal axis from the other two amplifiers. This is because the a-cut YLF crystal is birefringent along the light propagation direction in the a- and c-axes with different thermo-optical parameters, which results in the laser beam experiencing different values of the thermal lens in these two axes. By placing the second rod with orthogonal axis, the thermal lens difference between the two directions can be compensated.

In the free-running regime, a maximum output energy of 154.2 mJ and slope efficiency of 19.1% are achieved at 1 Hz under a pump energy of 2.15 J, while 94.2 mJ and 12.7% are achieved at 10 Hz. The laser efficiencies at maximum output reach 7.2% and 4.4%, respectively. The output energy increases with the increasing pump energy and decreases with the increasing of the repetition rate. No thermal rollover is observed. In the Q-switched regime, a maximum output energy of 59.5 mJ is obtained at 10 Hz, with a slope efficiency of 6.1%, a laser efficiency of 2.6%, and a free-running/Q-switching conversion ratio of 0.47. The output pulse duration decreases with pump energy and is measured as 133.8 ns at the maximum output energy. After the first stage amplifier, the pulse energy is increased from 58.5 mJ to 18 mJ under 3.52 J input energy and is further increased to 128 mJ and 201 mJ after the second stage amplifier and three stage amplifier, respectively. The amplification rates are 1.53, 1.43, and 1.57 for the first stage amplifier, second stage amplifier and three stage amplifier , respectively. A total amplification rate of 3.43 is achieved using this 3-stage double-pass PA. Regarding the beam quality, a triangle-shaped near-field beam is observed from the MO output, which is in line with the shape of its pump area in the laser head. After the first stage amplifier, this triangular shape is enhanced, while after the orthogonally mounted second stage amplifier, the beam edge softens. Since the first stage amplifier, second stage amplifier and three stage amplifier provide similar thermal lens values, the final output beam exhibits a near-circular shape with an ellipticity of 0.88.

Aiming at the amplification demands in the area of long-range wind Lidar, a MOPA based on (Tm, Ho) ∶YLF crystal rods is demonstrated to provide 2 μm laser sources with high energy and long pulse duration. In the MO, a maximum output energy of 59.5 mJ with 133.8 ns pulse duration and central wavelength of 2051 nm is achieved at 10 Hz in the Q-switched regime using 8-shaped ring cavity. In the 3-stage two-way PA, the pulse energy is amplified to 201 mJ, while the pulse duration is slightly narrowed to 131.8 ns. This MOPA reduces the required maximum pump energy/peak power in each stage and optimizes the output beam quality with a simplified setup using an orthogonal arrangement of the crystal axes in each stage. Moreover, in both the MO and PA , a self-designed triple side-pumped, conductive-cooled laser head module is employed. This module integrates LD bars, laser crystal rods, conductive-cooling heat sinks, and a wedged lens pump light-coupling system, demonstrating a highly compact and robust design. This study provides a novel laser source for future airborne and spaceborne Lidar systems.