# 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
Figure 1: Energy characteristics of Ti:Sapphire laser with VBG **(a)** and STG **(b)**. Reprinted with permission from ref. [8]. Copyright 2015 © The Optical Society. was restricted by the maximum EPUMP = 60 mJ, which was 50 Hz for our pump source. Due to the high LIDT, we could expect the successful operation of STG at much larger energy densities than occurred here. The Ti:Sapphire laser energy's behavior at 100 Hz was identical to at 50 Hz, but the maximum EOUT of the STG laser was restricted here to 15 mJ by the reduction in pumping Nd:YAG laser energy (this is why the maximum EPUMP was reduced to 53 mJ). The following experiments were performed at 100 Hz. Pulse duration of the STG laser at EOUT = 19 mJ was 17 ns, which increased to 30 ns at EOUT = 5 mJ. Therefore, the maximum pulse power here was 1.1 MW. Similar temporal characteristics were obtained for the VBG laser. The spectral line width, \(\Delta\lambda\), of the laser with STG, measured by a wavelength meter with a resolution limit \(\sim\)30 pm at \(\lambda\sim\) 1000 nm, was 33 +/\(-\) 2 pm at EOUT = 5 mJ, which increased to 48 +/\(-\) 2 pm at EOUT = 19 mJ. \(\Delta\lambda\) at the VBG laser did not exceed the wavelength meter resolution limit at any level of EOUT. We could estimate the effective spectral resolution, R1, of STG as the number of grating grooves illuminated by a laser beam. In our case, 1500 grooves/mm, a beam width of 0.7 mm, and an incidence angle of 44\({}^{\circ}\) obtained R1 \(\approx\) 1500. The spectral resolution of VBG, R2 = \(\lambda\)/\(\Delta\lambda\approx\) 9000, was much larger than R1. The real value of the VBG laser spectral width is supposed less than 30 pm. Wavelength deviation, \(\delta\lambda\), for the laser with STG did not exceed 10 pm during \(\sim\)5 min of operation at EOUT = 12 mJ. For the laser with VBG under the same conditions, we observed \(\delta\lambda\sim\) 30 pm. We assume that the primary reason for wavelength deviation in the last case is VBG heating. According to [6], the thermal shift of VBG central wavelength, induced by PTR glass thermal expansion, is \(\sim\)7 pm/K at \(\lambda\) = 850 nm. At our wavelength, 932 nm, thermal shift was 7 pm/K \(\times\) (932 nm/850 nm) \(\approx\) 7.7 pm/K. Therefore, the observed \(\delta\lambda\) corresponds to the VBG temperature increase \(<\)4 K, which may have been caused by room air heating. In comparison, the estimation of the STG wavelength shift induced by the thermal expansion of fused silica was \(\sim\)0.5 pm/K, and only \(\sim\)2 pm for a 4 K temperature increase. STG has a definite advantage over VBG for applications where laser generation with high wavelength stability is required. On the other hand, as mentioned in [6], the high sensitivity of the VBG central wavelength to temperature enables the fine tuning of the laser wavelength. Wavelength tuning of the STG laser was produced by rotating the output mirror under a fixed incidence angle at grating and fixed pumping levels. The results are shown in Figure 2. The obtained tuning range was 170 nm. This range was restricted at wavelengths shorter than 840 nm by a reduction in the rear laser mirror reflection coefficient. It is worth mentioning that STG has a low wavelength dependence of diffraction efficiency and can be specially designed with reduced diffraction efficiency, but increased wavelength range. Figure 2: Wavelength dependence of Ti:Sapphire laser energy (1) and reflection coefficient of rear mirror (2). Reprinted with permission from ref. [8]. Copyright 2015 © The Optical Society. THG was produced by two cascade LBO crystals: the first for SHG type I, and the second for THG type I, with a double wavelength wave plate after the first crystal to match 932 and 466 nm light polarizations for THG. Here, the efficiency of SHG was the same for both laser types, which exceeded 50% when E\({}_{932}>12\) mJ. The maximum THG efficiency of the VBG-type laser was 41%, close to the theoretically maximum value for the beams with a Gaussian profile.