The 2.1 μm holmium (Ho) laser has important applications in biomedicine, infrared optoelectronic antagonism, polymer material processing, and mid-far infrared nonlinear frequency conversion. Compared with mainstream in-band pumped Ho lasers, the intracavity pumped Ho laser can achieve efficient room-temperature Ho laser output based on the compact pump structure of conventional 800 nm laser diode (LD) without additional high-performance 1.9 μm thulium-doped (Tm) all-solid-state or fiber laser pump sources. In this Ho laser structure, thulium-doped and Ho-doped gain media are placed together in the resonant cavity, and 1.9 μm laser is generated in the resonant cavity under the pumping of Tm medium by conventional LD, and the Ho-doped medium is pumped in the same band. Compared with Tm and Ho co-doped lasers with conversion loss, this mechanism has a higher LD-Ho conversion efficiency at room temperature because there is no Tm laser leakage from the cavity during laser operation. In this study, by using Tm∶YLF crystal with negative thermal lens effect to alleviate the combined thermal lens effect of the Ho∶YVO4 laser pumped in the cavity, the highest Ho laser output power of 2052 nm is 3.3 W, the slant efficiency is 14.5%, and the LD-Ho laser photoconversion efficiency is 11%. The acavity-pumped Ho vanadate laser has the highest laser power and laser efficiency.
Polarization absorption spectra of Tm∶YLF crystal sample with Tm atomic fraction of 3% and Ho∶YVO4 crystal sample with Ho atomic fraction of 0.6% are measured using a ultraviolet-vision-near-infrared spectrophotometer. It is used to evaluate the spectral overlap between the absorption bands of the Tm∶YLF laser and the Ho∶YVO4 crystal in the cavity (Fig. 1). The designed intracavity pumped Ho∶YVO4 laser adopts a c-cut Tm∶YLF crystal with a size of 3 mm×3 mm×14 mm and an a-cut Ho∶YVO4 crystal with a size of 3 mm×3 mm×4 mm (Fig. 2). The 2 μm laser power is measured using a pyroelectric power meter. The laser wavelength is measured using a mid-infrared spectrometer. The beam quality of the Ho laser pumped into the cavity is measured using a beam quality (M2)analyzer.
A Tm∶YLF laser output of 11.38 W is obtained at 92 nm LD incident power of 30 W with a slope efficiency of 44.3% and light-to-light conversion efficiency of 37.9%. The laser center wavelength measured at the highest Tm laser power is 1909.7 nm, corresponding to the fluorescence emission band (σ polarization) along the a-axis of Tm∶YLF crystal (Fig. 3). At a pump power of 8.5 W under the wavelength of 792 nm, the laser oscillates and achieves the highest Ho∶YVO4 laser output of 3.3 W, and the corresponding LD-Ho laser optical conversion efficiency reaches 11% [Fig. 4(a)]. The fitting results show that the slope efficiency of the Ho laser reaches 14.5%, which is significantly higher than that of the Tm∶ YAP-pumping Ho∶YVO4 laser (10.4%). The leaky 1909 nm Tm∶YLF laser is detected near the threshold pumping power (8.5-9.8 W), and then the Tm laser signal disappears and Ho∶YVO4 laser starts to vibrate [Fig. 4(b)]. In the process of increasing the output power, the output wavelength of the Ho laser is stable at (2052.2±0.5)nm, and no residual Tm laser signal is detected. This phenomenon can be interpreted as follows: when the Ho laser starts to vibrate near the threshold value, the energy consumed by the Tm laser in the cavity is limited, and the Tm laser signal can be observed. With an increase in Ho laser power, the energy consumed by the Tm laser increases. At this point, the Tm laser enters a new steady state, and the gain generated by LD pumping is mainly balanced with the resonant absorption loss of the Ho∶YVO4 crystal. Compared to the strong Ho laser signal, the Tm laser leakage signal is so weak that it is drowned in spectral noise. The beam quality in the horizontal and vertical directions at the highest Ho laser power is 1.33 and 1.46, respectively (Fig. 5).
Tm∶YLF laser intracavity pumped Ho∶YVO4 laser with a compact structure and direct pumping by a conventional laser diode is reported. To achieve efficient overlap of the absorption band between the Tm∶YLF laser and Ho∶YVO4 crystal, a Tm∶YLF crystal cut along the c-axis is used to achieve the σ-polarized Tm laser output near 1909 nm, which is verified by the Ho laser pumped into the cavity. In the Tm∶YLF laser experiment, a maximum output of 11.3 W with 1910 nm Tm laser is achieved, with a corresponding slope efficiency of 44.3% and light-to-light conversion efficiency of 37.9%. The highest 2052 nm laser output power of 3.3 W is achieved with the intracavity pumped Ho∶YVO4 laser. Owing to the weak thermal lens effect of the Tm∶YLF crystal, intracavity pumped Ho∶YVO4 laser output is guaranteed with high beam quality. The above-mentioned results indicate that the room-temperature Ho laser can be pumped at the watt level from the Ho vanadate-doped crystal by the direct pumping of a conventional LD.
