Objective Owing to the advantages of being highly transmitted in the atmosphere, being safe to human eyes, being in the absorption spectrum band of water molecules, and being an efficient pump source for 3--5-μm laser, solid-state lasers emitting around 2 μm have been widely employed in many areas, including the medical field, high-precision ranging, environmental monitoring, and photoelectric counter-measurement. Laser source emitting at 2 μm with high energy and high beam quality output performance is a paramount component of the remote wind-measuring light radar. In addition, more than one hundred nanosecond pulse duration and narrow spectral linewidth are required. Therefore, the Q-switched 2-μm solid-state laser with a high energy output at room temperature has been a research hotspot for the space-borne lidar. Usually, the laser source used in the ground-based wind lidar system adopts the traditional liquid-cooling method, whereas those used in the air- or space-borne lidar system should adopt the conductive cooling design. To meet the special application requirement of the space-borne lidar, in this study, a laser diode (LD) side-pumped (Tm, Ho)∶YLF laser with a conductive cooling structure is developed by optimizing the key parameters of four-mirror ring cavity, such as the length of the cavity, the pump beam size inside the gain medium, output coupler mirror. As a result, high energy output with high energy efficiency and good beam quality are achieved.
Methods The special eight-shaped ring cavity comprising two concave mirrors and two plane mirrors is designed (Fig. 1). The pulse width of the Q-switched laser is calculated as a function of cavity length (Fig. 2). To obtain a Q-switched pulse output of more than one hundred pulse width, the length of the ring cavity is chosen about 2580 mm. In theory, there are two waist positions inside the ring cavity, one placed with acousto-optic modulation (AOM) and another with (Tm, Ho)∶YLF crystal. In experiments, the driving signal waveform of the AOM is a square wave pulse with a pulse duration of 4 μs. In addition, the LD side-pumped arrangement with a novel wedge waveguide lens is specially designed (Fig. 4). The crystal rod is side-pumped by two banks of five radically arranged LD modules, each is capable of outputting a maximum of 200-W pump power. The size of the lens duct is designed by ZEMAX software, and the energy distribution is calculated by MATLAB (Fig. 5). Almost 48% of the absorbed pumping energy by the gain crystal (the crystal radius is r=2 mm) is centralized in the central area of the rod (r<1.1 mm).
Results and Discussions At a repetition rate of 1 Hz, pulse energy and pulse width variation are recorded as functions of the pump energy input in the free-running and Q-switched modes (Fig. 6). A maximum output pulse energy of 340 mJ is obtained in the free-running mode with 3.3 J of pump energy input; the optical-optical efficiency is approximately 10.3%, and the slope efficiency is 22.2%. Meanwhile, the maximum output pulse energy of 141 mJ is obtained in the Q-switched mode with pump energy of 3.3 J; the optical-optical efficiency is 4.3%, and the slope efficiency is 8.3%. The output ratio of Q-switching to free-running reached up to 0.41. The Q-switched pulse waveform is detected by a photodetector with a 12.5-GHz sampling rate, and a pulse width of about 103 ns is detected (Fig. 7). Obviously, multilongitudinal mode lasing is demonstrated. The spectral linewidth is almost 0.82 nm (Fig. 8). The laser beam quality is measured using a lens with a focal length of 1 m and a CCD(Charge Coupled Device) camera; the spot diameter of the laser beam is recorded at different positions along the optic axis. Fitted by the Gaussian beam propagation equation, the beam quality factors of Mx2 and My2 are achieved as 1.22 and 1.08, respectively (Fig. 9). The inlet in Fig. 9 shows the far-field intensity distribution of the laser beam, indicating the output pulse is near the fundamental transverse mode.
Conclusions To meet the requirements of high pulse energy output at 2-μm spectral range for remote wind measurement in space-borne lidar, an acousto-optic Q-switched, conductively cooled (Tm, Ho)∶YLF laser emitting at 2051 nm is developed in this study. In a specially designed eight-shaped ring cavity, using an LD side-pumped arrangement with a novel wedge waveguide lens-coupling system, a Q-switched laser pulse output with more than one hundred nanosecond pulse width is achieved. At a repetition of 1 Hz and maximum pump energy of 3.3 J, a 141-mJ Q-switched laser with around 103-ns pulse width is obtained. The optical-optical and slope efficiencies are 4.3% and 8.3%, respectively. The central wavelength is 2051.4 nm, and beam quality factors of Mx2 and My2 are detected as 1.22 and 1.08, respectively. The experiment results are helpful to develop a high pulse energy of about 2-μm laser pulse output with a narrow linewidth by adopting a seeder injection method, which is needed to develop the high-performance coherent wind lidar for remote detection.
