# Engineering of Ti:Sapphire Lasers for Dermatology and Aesthetic Medicine
## 1 Introduction  
The application of Ti:Sapphire lasers in medical practice is an exception rather than a rule. Only a few devices are represented in the medical laser market at this time. However, the unique properties of Ti:Sapphire crystals, such as their very wide luminescence spectrum, high emission cross section and figure of merit, in combination with excellent thermal characteristics, are highly favorable for the development of specialized medical laser devices with advantages and competitive ability against similar devices, realized on the basis of other active media. Since 2012, our company, LASEROPTEK, has developed two medical laser devices, PALLAS and HELIOS-4, which include Ti:Sapphire lasers in combination with other modules, such as Nd:YAG lasers and nonlinear frequency converters. In this review, we omit a number of technical details and concentrate on new engineering solutions and on the results of laser physics studies obtained during the development of Ti:Sapphire lasers. Other related questions are described briefly to explain the actuality of the work and provide an overview of the devices. The first device, PALLAS, was developed to generate UV radiation with the characteristics required for the light treatment of dermatology-related diseases, first of all, for psoriasis and vitiligo. The most efficient skin treatment here can be obtained using narrowband UV-B light (wavelength range 311-312 nm) [1]. Previous laser devices, used in practical dermatology since 2000, are based on 308 nm excimer XeCl lasers [2]. This wavelength is in close enough proximity to the UV-B range, and the high efficacy of excimer lasers with applications to skin treatment has been confirmed by numerous medical researchers [3]. In this regard, the serious disadvantage of the XeCl excimer laser is that the cost of consumables is very expensive. For instance, its after-sales support services include the frequent exchange of gas mixtures, cavity mirrors, windows and discharge electrodes, and it uses chlorine gas, which is highly reactive. Finally, annual service expenses costs more than 10% of the initial laser cost in Korea. From this point of view, the development of alternative lasers with low-cost consumables, based on solid-state lasers, is very important for users considering additional costs that can be saved. The typical characteristics of excimer laser light pulses for skin treatment [3] include the following: pulse energy 3-6.5 mJ, pulse duration 10-50 ns, and pulse repetition rate 100-400 Hz. Such characteristics of XeCl laser were used as a base for PALLAS development. It is worth mentioning that the potential level of the XeCl laser's pulse energy is much larger, exceeding 100 mJ, but such high energy levels are not suitable for skin treatment. It is possible to obtain the light with the required characteristics by the third harmonic generation (THG) of the Ti:Sapphire laser, operating at ~930 nm. The most advanced results in the generation of light at 300-315 nm on the basis of Ti:Sapphire lasers were obtained in 1999-2003 during the development of atmospheric ozone lidars [4; 5]. They showed that the efficiency of THG by LBO crystals for ~1.0 MW pulses with nanosecond durations can exceed 30% if the laser generation is in single mode and the spectral width is less than 0.1 nm. To obtain the required spectral characteristics of the Ti:Sapphire laser, authors [4; 5] used the injection seeding of narrow-line diode laser radiation in combination with preselection using a set of 4-5 dispersive prisms. It is also worth noting that well-known optical components, such as dispersion prisms, ruled metallic or holographic polymer diffraction gratings, and etalon Fabri-Perot, do not possess a total set of parameters, required for the successful realization of the abovementioned characteristics in Ti: Sapphire lasers.