Ho-doped gain media, which feature absorption and emission transition between manifolds ⁵I₈ and ⁵I₇, have longer lasing wavelengths (2 μm) and larger emission cross-sections than Tm-doped gain media. Thus, Ho lasers are more advantageous in various applications such as mid-infrared nonlinear frequency generation, photo-electronic countermeasures, medical treatment, and laser-range identification. However, high power Ho lasers with high slope efficiencies (SEs) can be obtained by in-band pumping 1.9 μm Tm laser sources. Nonetheless, the required system is large and expensive, which renders it impractical and inaccessible in many applications. In this study, an efficient Ho∶YAP laser pumped with a composite Tm∶YAG slab laser intra-cavity is demonstrated. The results provide an effective scheme to directly use typical laser diodes (LDs) to achieve a compact, accessible, linearly-polarized 2.1 μm laser at room temperature.

A fiber-coupled 785 nm LD, with a core diameter of 400 μm and a numerical aperture (NA) of 0.22, is used as a pump source to pump the absorption peak of a Tm∶YAG crystal. Plano-convex lenses F1 and F2, which feature identical focal lengths of 40 mm, are utilized to collimate and focus the pump light into a spot with a diameter of 380 μm within the Tm∶YAG crystal. Additionally, a Tm∶YAG crystal with a cross-section measuring 1.5 mm×6.0 mm and a length of 17 mm is applied. As more severe thermal effects occur in the Tm-doped gain medium during intra-cavity pumping, the pump end of the Tm∶YAG crystal is diffusion boned with another 3 mm long YAG slice to alleviate the thermal effects. Two Ho∶YAP crystals measuring 3 mm×3 mm×7 mm are applied, which are cut along the crystallographic a- and c-axes, to compare the Ho laser performance. A 42-mm-long plano-concave resonator is developed for intra-cavity pumping. The beam quality is evaluated using a pyroelectric camera, and the laser wavelength is measured using a spectral analyzer.

Efficient Tm laser operation with a maximum output power of 18.62 W is obtained with an SE of 49.82%, which corresponds to an optical conversion efficiency (OE) of 46.8% [Fig. 2(a)]. Using the a-cut Ho∶YAP crystal, a lower output power of 9.48 W at 2118.5 nm (E∥c) with an SE of 26.1% and an OE of 23% is obtained [Fig. 4(a)]. Based on Fig. 3, the laser wavelength for the a-cut Ho∶YAP laser is 2118.5 nm [measured at an output power of 9.48 W, Fig. 5(a)], with the polarization direction being parallel to the crystallographic c-axis. At the maximum Ho laser power, the beam quality is measured and fitted with beam quality factors of 1.81 and 1.94 along the transverse and vertical directions, respectively [Fig. 5(b)]. A maximum Ho laser power of 12.07 W at 2129 nm (E∥b) is obtained using the c-cut Ho∶YAP crystal, with an SE of 32.2% and an OE of 28.7% [Fig. 6(a)]. At the maximum output power of 12.07 W, the laser wavelength is measured to be 2129.61 nm [E∥b, with a polarized extinction ratio of 18.3 dB, Fig. 7(a)], and the beam quality is measured and fitted with beam quality factors of 3.56 and 3.23 in the transverse and vertical directions, respectively [Fig. 7(b)]. Considering the maximum incident pump power of 42 W, the OEs for the current a- and c-cut Ho∶YAP lasers are 23.04% and 28.7%, respectively. This is because the intra-cavity absorptions for both Ho∶YAP crystals at 2022 nm are almost identical [Fig. 3(b)]. This difference in lasing efficiency is attributable to the cavity loss. By expanding the absorption spectra of the Ho∶YAP crystal at its lasing band (Fig. 8), we discover an absorption peak at approximately 2117 nm for the polarization along the c-axis, which results in a higher cavity loss for the a-cut Ho∶YAP laser. By contrast, no absorption peak is observed along the c-axis at approximately 2129.6 nm for the c-cut Ho∶YAP crystal. However, the c-cut Ho∶YAP laser, which shares the same crystallographic b-axis with the a-cut Ho∶YAP laser and exhibits a lower thermal conductivity along the a-axis [11.6 W/(m^-1·K^-1)] than along the c-axis [12.3 W/(m^-1·K^-1)] , shows worse beam quality.

A composite Tm∶YAG slab laser intra-cavity pumped Ho∶YAP laser with superior linearly polarized operation compared with YAG-based intra-cavity pumped lasers is presented. Maximum output power levels of 9.48 W at 2118.5 nm (E∥c) and 12.07 W at 2129.6 nm (E∥b) are obtained from a- and c-cut Ho∶YAP lasers respectively, with corresponding SEs of 26.1% and 32.2% and OEs of 23% and 28.7%, respectively. The lower lasing efficiencies of the a-cut Ho∶YAP laser is attributed to the presence of a re-absorption peak along the crystallographic a-axis at 2117 nm. The results provide a direct diode-pumped, linearly polarized Ho laser scheme with a compact structure, where higher lasing power is limited by the available incident LD power, thus obviating the necessity to develop another high-power 1.9 μm laser source for in-band pumping.