12.1 W,2129 nm Ho∶YAP Laser Based on Tm∶YAG Laser Intra-Cavity Pumping
Objective 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.Methods 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.Results and Discussions 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.Conclusions 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.
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