700800 nm lasers at the border of visible light and near-infrared light have been used in many fields such as nonlinear optics, remote sensing detection, laser jamming, and laser medicine. They including many different types of lasers such as dye lasers, Ti: sapphire lasers, and Alexandrite lasers. Dye lasers are developed very early, but their gain media are toxic and the maintenance of laser systems is difficult. The development of Ti: sapphire lasers is relatively mature. Because of the broad gain bandwidth of Ti: sapphire, it is mostly used in the field of ultrafast lasers. The pump source of Ti: sapphire is mainly expensive green lasers, such as 532 nm frequency-doubled Nd∶YAG laser. In addition, the thermal effect of the gain medium is serious. In contrast, Alexandrite lasers can be pumped by more economical red laser diodes (LDs) because of its broad absorption spectrum. Wavelength tuning can be achieved at room temperature and the laser performance of Alexandrite increases at elevated temperatures due to its unique spectroscopic properties. Alexandrite has a long fluorescence lifetime of about 262 μs at room temperature, almost 85 times longer than that of Ti: sapphire (about 3 μs). This causes tremendous advantages of energy storage at the upper level to achieve Q-switched operation and obtain a high peak power output. Although the emission cross section is only 0.7×10^-20 cm², the high damage threshold (270 J/cm²) provides the possibility for an Alexandrite laser to work with a high pump power. An Alexandrite laser is an ideal choice to obtain a laser output at 700800 nm. In addition, a high power red LD is developing in recent years, and it is an excellent pump source to obtain an Alexandrite laser with high power and high pulse energy.

Electro-optical Q-switched operations are used in our experiments. First, a V-shaped cavity is obtained and the length of the cavity is about 33 cm. A fiber-coupled red diode laser emitting at 638 nm with a maximum pump power of 40 W is used as the pump source. The size of an Alexandrite crystal is 3 mm×3 mm×10 mm with c-axis-cut, and it is wrapped with an indium foil and mounted in a water-cooled heat sink. By controlling the β_-BaB₂O₄ (BBO) Pockels cell, a pulsed laser with a multi-kHz repetition rate is obtained. Wavelength tuning is demonstrated with birefringent filters. Then, the output power, pulse duration, pulse energy, and peak power at the wavelength corresponding to the maximum output power are analyzed. Based on the Q-switched rate equations, the variation of pulse duration at a high pump power is predicted. To obtained shorter pulse duration, cavity dumping is considered. In addition, the performances of an Alexandrite laser at different repetition rates are analyzed in our experiments.

The schematic of an Alexandrite laser and its photograph are shown in Fig. 1. At the repetition rate of 10 kHz, a watt-level laser output is obtained with a pulse duration of 961 ns and a pulse energy of 116 μJ. Wavelength tuning outputs from 728 nm to 793 nm are obtained at repetition rats of 5 kHz and 10 kHz [Fig. 2 (a)] with the combination of birefringent filters (BRFs) in different thicknesses, and the wavelength corresponding to the maximum output power is 755 nm. The pulse characteristics including output power, pulse duration, pulse energy, and peak power at 755 nm with 10 kHz and 5 kHz repetition rates are analyzed (Fig. 3). Based on the Q-switched rate equations, a well-matched numerical simulation is obtained. The variation of pulse duration with absorbed pump power is simulated (Fig. 4). The predicted shortest pulse duration at a high enough pump power is about 50 ns. Furthermore, a short pulsed laser of 10 ns is achieved by cavity dumping [Fig. 5 (b)], whose peak power is over 3 kW.

This paper presents a red LD pumped Q-switched Alexandrite laser. A watt-level pulsed laser is obtained in a V-shaped cavity. The corresponding repetition rate is 10 kHz, the pulse duration is 961 ns, and the pulse energy is 116 μJ. Wavelength tuning from 728 to 793 nm is demonstrated with BRF. Based on the Q-switched rate equations, a well-matched numerical simulation is obtained, and the effect of pump energy on pulse duration is analyzed. A shorter pulse laser of 10 ns is also achieved by cavity dumping and the peak power is over 3 kW. In the future work, we will continue to optimize the design of the cavity and the experimental parameters. Based on the need of specific applications, we will try to achieve higher-energy, higher-efficiency, shorter pulse duration, and narrower linewidth Alexandrite lasers pumped by red LDs.