# Engineering of Ti:Sapphire Lasers for Dermatology and Aesthetic Medicine
## 1 Introduction
During our work, we evaluated the application of new types of diffraction gratings: photo-thermo-refractive (PTR) glass volume Bragg gratings (VBG) [6] and fused silica subwavelength surface relief transmission gratings (STG) [7]. The original results of this study, which were obtained for the first time, were published in [8; 9]. We review them in the following Section 2.1. The second device, HELIOS-4, which includes the Ti:Sapphire laser, was developed for application in tatto removal (TR) procedures. The popularity of laser TR is growing, and the present demand for lasers for TR is already significant. Preliminary studies in laser TR began in the mid-1960s and has been the most common method in clinical practice since the 1990s. Standard TR procedures at that time used Q-switched lasers with nanosecond pulse duration [10; 11]. In 1998, it was shown that the usage of lasers with subnanosecond pulse duration considerably increases TR efficacy in comparison to treatment by nanosecond lasers [12]. The advantages of TR using the 795 nm radiation of the experimental Ti:Sapphire laser with a 500 ps pulse duration were demonstrated as far back as 1999 [13]. Therefore, it is surprising that the first TR device generating subnanosecond pulses, PICOSURE (from CYNOSURE), was developed only 16 years later, which obtained FDA approval in 2012. PICOSURE is based on the Cr:Alexandrite laser, operating at 755 nm with a pulse duration of 550-750 ps, maximum pulse energy of 200 mJ and pulse repetition rate of 10 Hz [14]. After 2012, several TR devices, based on Nd:YAG lasers with subnanosecond pulse duration, such as PICOWAY (CANDELA-SYNERON) and CUTERA (ENLIGHTEN), generating at 1064 and 532 nm, were developed and FDA approved. However, no other subnanosecond TR lasers generating radiation at 755-795 nm were developed until 2016. The principle of subnanosecond pulse generation in PICOSURE is active mode-locking. It is notable that the Ti:Sapphire laser, used in [13], is operated by the same principle. In PICOSURE, mode-locking is combined with cavity dumping, which are both produced by an original electro-optical modulator with an original high-voltage fast electronic driver. To increase the active volume and obtain a relatively large (for mode-locked lasers) pulse energy, PICOSURE operates at several transversal modes. This leads to the essential deterioration of laser stability, which is much worse than that of the Nd:YAG lasers mentioned above, based on more simple principles of operation. It is equally important to note that the disadvantage of PICOSURE is its high price, in particular, due to the high cost of an electro-optic system for mode-locking and cavity dumping. As a result, in 2012, the price of PICOSURE exceeded USD 200,000. Therefore, the development of simple and lower-cost subnanosecond TR lasers, which possess better stability and operate at 750-795 nm, also presented a problem. In 2001, scientists from the Massachusetts Institute of Technology showed that single subnanosecond pulses can be generated in gain-switched Ti:Sapphire lasers with very short laser cavities [15] under pumping using pulses with a small enough duration. Pumping by the 532 nm radiation of Nd:YAG microlaser with a pulse duration of 600 ps, they obtained Ti:Sapphire laser generation at 775 nm with the energy of 18 \(\mu\)J and pulse duration of 350 ps. The Ti:Sapphire laser cavity length was 3.5 mm. Following this approach, in 2016, CANDELA-SYNERON was developed and represented a new medical gain-switched subnanosecond Ti:Sapphire laser as an accessory for PICOWAY [16]. The Ti:Sapphire laser was made as a small cylinder, which can be embedded into the handpiece at the output of the PICOWAY articulated arm. It was pumped by 532 nm subnanosecond pulses of PICOWAY and generated light at 785 nm with a maximum energy of 130 mJ and pulse duration of 300 ps. Based on this, PICOWAY is a simpler and more economical solution than PICOSURE, but not in terms of maximum pulse energy.