PROPERTIES AND MEDICAL APPLICATIONS OF NEAR-IR SOLID-STATE LASERS

Cr-Tm-YAG, Cr-Tm-Ho-YAG, and Er-YAG lasers at 2.01 pm, 2.1 2pm, and 2.94 pm, resp., are compared regarding single-shot and high-repetition rate efficiency. The 2 pm lasers are intrinsically limited at higher operation temperatures, the Er-laser only shows thermal lensing. Th~n (3 mm) Tmand Ho-laser rods allow high repetition rates (30 Hz) and with increasing rod diameters pulse energies up to 17 J are obtained. Flashlamp-pumped near-IR solid-state lasers based on Tmh 11 1, Ho* 12,3,4(, and E P 51 doped YAG have the potential for many applications, especially in medicine. However, t b e operation characteristics of the laser ions require spec~al optimization of the laser set-up for different applications. We present an overview of the laser performances of Tm-, Hoand Er-lasers that gives data necessary for this optimization. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991701 JOURNAL DE PHYSIQUE IV The Cr-Tm-laser emits at 2.01 pm, the Cr-Trn-Ho-laser at 2.1 2 pm. The Cr ion acts as a broad band absorber of the flashlamp continuum radiation and effectively transfers the energy to the Tm ions. A cross-relaxation process results in two Tm ions excited into the upper laser level 3H for every absorbed pump photon. The Ho ion is directly excited into the upper laser level$, by non-radiative transfer from the 3H,Trn level. The laser transitions for Trn and Ho terminate in the highest levels of the ground state multipleits, thus both are "quasi-3-level" lasers. Typical single-shot efficiencies are given in Fig. 1 foraclose-coupledsingle-lampcerarnical reflector. The behaviourfor higher repetition rates (Fig. 2) shows a temperature dependence of the laser output for Ho and Trn. Increasing the rod temperature by higher mean pump owers decreases the gain by higher reabsorption from the thermally populated lower laser eve1 and by lower Boltzmann population of the upper laser level. For the Ho-laser, in P addition, the stored energy distribution between the Tm and Ho ions is shifted towards Tm decreasing the Ho-laser efficiency. Optimization for high repetition rates requires thin laser rods: with 2.8 mm diameter Cr,Tm,Ho-rods 30 Hz operation is possible 161. On the other hand, single-shot high output energies up to 17 J are obtained with 6 mrn diameter Cr,Tm,Ho-rods (41. Fig. 1 .: Single-shot operation of Cr-Tmand Cr-Trn-Ho-YAG, 3 Hz-operation of Er-YAG (rods 4 X 80 mm without AR-coating, pulselength 500 ps, T=lO% (Ho,Er), 20% (Tm)) Fig. 2.: Relative pulse energies of Cr-Tmand Cr-Trn-Ho-YAG for variable repetition rates (parameters as in Fig. 1 .) 3. Fr-YAG Laser The 2.9 pm laser operates between the 4111, and 41, levels being a 4-level-system. The efficiency is lower than for the Tmand %lo-lasers because of the longer emission wavelength and a missing dominant cross-relaxation excitation process as in Tm. The 4level-system does not have an intrinsic temperature dependent efficiency. The observed decrease of the output energies at higher repetition rates (Fig. 3) is ascribed to thermal lensing, because applying AR coatings to the endfaces of the laser rod cancels this effect. By inserting two CaF2-prisms into the resonator the emission can be tuned to six lines in the long-wavelength part of the 4l -> 411,fluorescence: 2.80,2.83,2.86,2.87,2.90, and 2.94 pm. Shorter wavelengths do not oscillate, because the corresponding transitions terminate in the lower states of the 4 1 , , multiplett which are populated by the long lifetime of this level. The 2.83 pm and 2.94 pm lines have comparable efficiencies, the others are 3 10 times weaker.