High-energy and high-peak-power 2 μm solid-state laser plays an important role in many fields. Currently, two types of pumping source can be used to pump an Ho∶YLF laser. One is the Tm doped solid-state laser, and the other is the Tm doped-fiber laser. It is known that Ho∶YLF crystal has different polarized absorption characteristics in the π and σ directions. Therefore, quantitative research on the optical-optical conversion efficiencies affected by the polarization of the pump light will be remarkably significant. Electro- and acousto-optic Q-switching are two methods for realizing pulsed output of Ho∶YLF laser. The former method has advantages in terms narrower-pulse-width laser output. Therefore, we demonstrate an electro-optic Q-switched Ho∶YLF laser pumped by Tm∶YAP laser, and a laser output with high-peak-power and narrow-pulse-width is obtained. A peak power of megawatt level is achieved at a lower pump power by adopting the technical scheme. Moreover, the laser has a compact configuration. It will potentially provide a high-quality laser source for the application of mid- and long-infrared optical parametric oscillators, laser radar, and other fields.
This study develops layouts of the Ho-laser based on an end-pumped structure with L-shape resonator. The experimental device is depicted in Fig. 2. A home-made Tm∶YAP solid-state laser at wavelength 1.94 μm is used as a pumping source. The dimensions of the Tm∶YAP crystal are 3 mm×3 mm×12 mm, and the doping concentration (atomic fraction) is 3.0%. The Tm∶YAP crystal is b-cut. The dimensions of the Ho∶YLF crystal are 6 mm×6 mm×40 mm. The Ho∶YLF crystal is a-cut, and the doping concentration (atomic fraction) is 0.5%. The M4 mirror is a half-wave plate and is used to examine the effect of polarization on the output characteristics of the Ho∶YLF laser. To achieve good mode matching, the size of the pump light spot is adjusted to be approximately 1.2 times the size of the Ho∶YLF laser spot. We select an RbTiOPO4 (RTP) crystal as the electro-optic Q-switch for achieving high-peak-power laser output. Further, we adopt a long-pulse Tm∶YAP laser to pump the Ho∶YLF crystal to obtain high beam quality while the output energy is adjusted. To study the effect of different pump pulse energy on the characteristics of the laser output, the laser beam quality factors are measured using the knife-edge method.
The Ho∶YLF crystal is pumped by a linear-polarized Tm∶YAP solid state laser with a power of 30.2 W. Under continuous-wave operation, the central wavelength of Ho∶YLF laser is 2.06 μm, and the beam quality factors are 1.3 and 1.2 in the horizontal and vertical directions, respectively. The spot sizes at different positions and beam shape are shown in Fig.6. A half-wave plate is placed after the Tm∶YAP laser, which is used to change the polarized direction of the pump light. As the proportion of the pump laser in the direction of π polarization changes from 100% to 0, the optical-to-optical conversion efficiency changes from 28.9% to 7.1%. This indicates that the efficiency can be significantly improved by adopting linearly-polarized pumping light. The experimental results are shown in Fig. 7. Based on electro-optical Q-switching technology, a pulse laser output is obtained when the Tm∶YAP laser power is 15.4 W and repetition frequency is 100 Hz. The maximal output energy is 9.5 mJ, with a pulse width of 13.0 ns, and peak power is 0.73 MW. The pulse waveform and change of pulse energy with the pump energy are shown in Fig. 8.
In this study, an Ho∶YLF solid-state laser and the corresponding Tm∶YAP pump laser are built, and a pulsed laser with peak megawatt power at 100 Hz is realized. The effect of the polarized characteristics of the pump source on the output characteristics of the Ho∶YLF laser is studied; the results indicate that linear-polarized light pumping is an effective manner to improve the optical-to-optical efficiency of an Ho∶YLF laser. The laser has the advantages of compact configuration and high-peak power and therefore has broad application prospects in the field of optical parametric oscillator pumping. Furthermore, the results of electro-optical Q-switching under long pulse pumping also provide solid foundations for Ho-doped lasers with higher energy and higher peak power.
