# Engineering of Ti:Sapphire Lasers for Dermatology and Aesthetic Medicine
## 2 Experimental Results and Discussion
### The Characteristics of Short Cavity Ti:Sapphire Gain-Switched Laser under Nanosecond Pumping  
To achieve the generation of single subnanosecond pulses by gain switching, the duration of pump pulse should be less than the build-up time of a generated pulse. Build-up time, T\({}_{\text{D}}\), and the duration of a generated pulse T reduce when pump fluence increases. When T\({}_{\text{D}}\) becomes less than the duration of the pump pulse, pumping will continue after the appearance of the first gain-switched laser pulse. The subsequent increase in pump fluence will lead to second pulse generation. This process restricts the maximum useable pump fluence and minimum gain-switched laser pulse duration. The theory in [20] predicts that the minimum duration of generated pulse can be several times shorter than the duration of pump pulse. In this study, for pumping, we used 532 nm radiation in our standard laser device, HELIOS-3. It consists of an electro-optically Q-switched multimode Nd:YAG master oscillator, followed by an amplifier and second harmonic generator. The maximum pulse energy at 532 nm here was 700 mJ with a pulse duration of 6 ns. We modified the resonator of the master oscillator and reduced the 532 nm pulse duration to 4 ns. Figure 3: PALLAS laser head: 1 is Ti:Sapphire crystal (in the holder); 2 and 3 are total and output mirrors, respectively; 4 is STG. Using data [15], we could estimate the average value of pump fluence, \(W_{\text{PUMP}}=E_{\text{PUMP}}/S_{\text{PUMP}}\), required to reduce the Ti:Sapphire laser pulse duration to below 1 ns: \(W_{\text{PUMP}}\sim 1\) J/cm\({}^{2}\). Here, \(E_{\text{PUMP}}\) represents the total pump energy, and \(S_{\text{PUMP}}\) is the pump spot area. Therefore, to obtain a laser pulse energy larger than that of PICOSURE, E \(\geq\) 200 mJ, the linear size of the square pump spot inside the Ti:Sapphire crystal had to exceed 4-5 mm. An optical scheme of Ti:Sapphire laser is shown in Figure 4. Ti:Sapphire active crystals had an absorption coefficient of 3.5-4.0 cm\({}^{-1}\) at 532 and a thickness of 3 mm; they were AR-coated at 532 and 780-820 nm. The pump mirror had 96% transmission at 532 and 99.5% reflection within 780-820 nm. We used output mirrors with reflection from 40% to 75% to generate wavelengths. The output mirror had an additional high reflective coating at 532 nm to increase the absorption of pump radiation in the Ti:Sapphire crystal by reverse pass. Both cavity mirrors were flat. The distance between mirrors was 3.5-7.5 mm. Laser pulse duration, T, was determined using a photodiode (UPD-50-UD, rise time <40 ps and fall time 50 ps, from Alphalas), in combination with a digital oscilloscope (Infiniium DSO 80804B, 8 GHz bandwidth, from Agilent). Pulse duration was determined for the light integrated from the whole laser aperture. For this purpose, a diffuser was placed at 50 cm from the Ti:Sapphire laser output mirror, and then the laser spot at the diffuser was imaged on the photodiode. At the initial stage of the work, without homogenization, we directly focused 532 nm multimode pump radiation into a spot with the required size at the surface of the Ti:Sapphire crystal. The first results were disappointing: generated pulses have very unstable shape, varying from pulse to pulse, and the duration was larger than 900 ps (see Figure 5). Similar characteristics were obtained via pumping using a singlemode Nd:YAG laser. To explain these results, we proceeded from the assumption that a laser with a very short cavity and relatively large lateral size of active area (Fresnel number \(N_{\text{F}}\geq 1000\)), where no physical reasons exist for the coupling of a laser field in the lateral direction, can act as an array of multiple independent local channels with different lateral sizes and shapes and generate in parallel. In the case of flat mirrors, all channels have equal losses, but different gains, as determined by the local value of pump fluence. If the local gain is different, then the T and T\({}_{\text{D}}\) of the local channels will also be different.