Objective Water has a strong absorption coefficient in the mid-infrared band, which explains why mid-infrared lasers are widely used in medicine. The thulium-doped fiber laser (TFL) emission wavelength range includes the water absorption peak at 1940 nm, which has been used in prostatic hyperplasia tissue resection surgery. There have been numerous reports on TFL being used in lithotripsy, including the mechanism of action and clinical tests. There was, however, no mention of a long pulse mode or a high pulse repetition rate mode. We developed a quasi-continuous wave (QCW) TFL for laser lithotripsy to address these issues. In conjunction with previous reports, this study focuses on the use of various laser parameters to ablate calculus in vitro in order to verify the feasibility of TFL laser lithotripsy and evaluate calculus ablation efficiency. Additionally, a thermometer is used to monitor the peak temperature of the water to ensure that the system is operating safely. Methods Six 140 W, 793 nm fiber-coupled laser diodes (LDs) pumped the QCW TFL, with the six LDs modulated synchronously by the drive current signal. The pulse width and repetition rate (PRR) of the LD drive current can be continuously tuned in the range of 0-2 ms and 0-2 kHz. Two fiber Bragg gratings (FBG) were used to construct the reflectance of laser oscillator. The reflectance of high-reflective FBG is 99. 8%, while the reflectance of low-reflective FBG is 9.38%. The gain fiber was a 4-m-long thulium-doped fiber with a core diameter of 25 mu m and an inner cladding diameter of 400 mu m. At 793 nm, the cladding absorption is 4.2 dB/m. The laser output port was a SMA905 connector that could be butt joined to a medical fiber via a converter. To simulate the clinical environment for laser lithotripsy, the calculus was placed in water, as shown in Fig. 4. Calcium sulfate was used as calculus in the experiment to investigate the effect of TFL parameters on calculus ablation efficiency and ambient temperatures. Three times before and after laser exposure, the mass loss of the calculus was determined. Finally, a thermometer was used to monitor the temperature of the water at a depth of 3 mm below the surface of the calculus. Results and Discussions The TFL laser had a wavelength of 1939.31 nm and a maximum average output power of 34.2 W. After modulation, the single pulse profiles and pulse trains were stable (Fig. 3). The experimental phenomena of laser lithotripsy revealed that when the medical fiber tip struck the surface of a calculus, it immediately produced a large amount of powder that diffused into the water, and the calculus' s surface had some burnt marks. Consequently, we conclude that there is an obvious photo thermal ablation mechanism in the process of TFL laser lithotripsy in a water environment. Table 2 displayed the measured mass loss of the calculus with various laser parameters. Mass loss increased by 0.09 g (200 Hz/9.53 W/0.047 J) up to 0.29 g (400 Hz/20.29 W/0.05 J) and 0.333 g (600 Hz/31.8 W/0.053 J) at a pulse width of 250 mu s. At first, the mass loss increased with increasing output power at pulse widths of 500, 1000, and 2000 mu s. Consequently, whether the laser pulse is long or short, the ablating rate increases with increasing laser power. Furthermore, the mass loss of ablation increased with larger single pulse energy while output power remained approximate, such as 0.333 g at 31.8 W/0.053 J (250 mu s), 0.480 g at 33.1 W/0. 11 J (500 mu s), 0. 697 g caused by 33.5 W/0. 22 J (1000 mu s), and 0. 723 g at 34.2 W/0. 45 J (2000 mu s). In this case, the single pulse energy was the primary influencing factor in calculus ablation. Moreover, the single pulse energy had an effect on not only ablation efficiency but also water temperature. When single pulse energy reached 0.45 J (2000 mu s/34.2 W) during laser irradiation, the peak temperature of water was 41.2 degrees C. As a result, for the same output power, the single pulse energy produced more heat in the same amount of time. Conclusions QCW TFL was constructed with a center wavelength of PRR of 1939. 31 nm, a pulse width of 0-2000 mu s, and a PRR of 0-2 kHz, respectively. 34. 2 W was the maximum average output power. Following that, in vitro lithotripsy experiments were performed using the laser. The results indicate that when the single pulse energy is constant, the average output power of the laser is the primary factor affecting the ablation efficiency of lithotripsy. And when the average output power is comparable, the energy of the single pulse becomes the determining factor. Additionally, the TFL can generate more heat when operating in the long pulse width mode, resulting in a greater rise in water temperature. In conclusion, TFL with a wavelength of 1940 nm has a significant lithotripsy effect. When combined with fiber lasers' superior characteristics, it is very likely to become the laser source for the next generation of laser lithotripsy.
