# Engineering of Ti:Sapphire Lasers for Dermatology and Aesthetic Medicine
## 2 Experimental Results and Discussion  
### Investigation of Ti:Sapphire Laser Operation with VBG and STG  
VBG is holographic diffraction grating, which can operate as an output coupler of the laser cavity and produce back reflection at a given wavelength with a narrow spectral bandwidth. PTR glass, a VBG material, possesses improved thermo-optical characteristics and a larger laser-induced damage threshold (LIDT) than in polymer holographic gratings. STG is a wide class of diffraction gratings with deep subwavelength surface relief, which are produced by photolithographic technology with the etching of fused-silica substrates. In addition, some dielectric coatings can be deposited onto these substrates over surface relief to increase diffraction efficiency and make gratings polarization insensitive. Unfortunately, such coatings considerably reduce the LIDT of the component. We used TE-type STG without similar coatings (with only an AR coating on a flat rear surface, which has high LIDT). In addition to high LIDT, the spectral dispersion of such gratings is about two times larger than the dispersion of standard ruled gratings due to the smaller (subwavelength) groove period. The parameters of the used gratings are listed in Table 1. We must clarify that the diffraction efficiency of our VBG, 54%, is made to correspond to the optimum reflection of the output coupler for the given operational conditions of the Ti:Sapphire laser. Passive losses in VBG, inserted into the laser cavity, are negligible. On the contrary, STG operatesin the Ti:Sapphire laser cavity as a bending mirror. This is why passive losses, inserted by FSTG and determined by radiation, transmitted into \"0T\"-order and reflected into \"-1R\"-order, are 100% \(-\) 95% = 5%. As a result, energy efficiency obtained for the Ti:Sapphire laser with VBG is larger than that for the laser with FSTG. The energy, spectral and spatial characteristics of the Ti:Sapphire laser were investigated with both types of gratings, with the laser operating at 50-100 Hz. Pumping of the Ti:Sapphire laser was conducted using the second harmonic of flash lamp-pumped electro-optically Q-switched Nd:YAG laser, which is capable of generating pulses with maximum E\({}_{1064}\) = 80 mJ at 100 Hz and \(-\)95 mJ at 50 Hz. SHG was performed using a KTP crystal with an efficiency of more than 60%, so the maximum available 532 nm pump energy, E\({}_{\text{PUMP}}\), at 100 Hz was 53 mJ and 60 mJ at 50 Hz. For a correct comparison of the characteristics, all laser cavity parameters and pump spot sizes for the Ti:Sapphire crystal were made equal for both of the used gratings. The dependence of output energy, E\({}_{\text{OUT}}\), at 932 nm on pump energy, E\({}_{\text{PUMP}}\), at 50 Hz is shown in Figure 1.  
The maximum laser efficiency obtained here with VBG was 35%, which is larger than the 32% obtained with STG. However, the maximum value of E\({}_{\text{OUT}}\) = 19 mJ for the FSTG laser was larger than the maximum E\({}_{\text{OUT}}\) = 14.5 mJ of the VBG laser. Energy restriction of the VBG laser was caused by VBG damage following a few seconds of operation at the indicated energy level. The estimated LIDT of these gratings did not exceed 15 J/cm\({}^{2}\). On the contrary, STG was not damaged during our study. The maximum E\({}_{\text{OUT}}\) in this case \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **STG** & **VBG** \\ \hline Operational diffraction order & -1T & 1R \\ \hline Wavelength range & 830–1000 nm & Single wavelength, 933 nm \\ \hline Angular dispersion & 2 mrad/nm & - \\ \hline Spectral bandwidth & - & 0.15 nm \\ \hline Diffraction efficiency & 95\% & 54\% (max \textgreater{} 99\%) \\ \hline Optimum angle of incidence & 44 deg & Normal incidence \\ \hline Laser damage threshold (estimated) & \textgreater{}30 J/cm\({}^{2}\) & \textless{}15 J/cm\({}^{2}\) \\ for 17 ns pulses & \textgreater{}30 J/cm\({}^{2}\) & \textless{}15 J/cm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of diffraction gratings.