Dermatological lasers are broadly classified as ablative or non-ablative, with tissue absorbance being a key consideration. The primary target in the Infrared (IR) spectrum is water, owing to its high absorption and its high concentration in tissues. Ablative lasers at 10 and 3μm, CO2 and Erbium:YAG lasers, respectively, leverage this characteristic effectively. Conversely, non-ablative, lasers at 1.5-2μm primarily coagulate tissues without achieving the ablation threshold. Thulium lasers are positioned around a local peak in water absorption at 1.94μm, and exhibits approximately six times lower absorption than CO2 (10.6μm) and significantly higher absorption than the 1.5μm wavelength. So far they were employed as more superficial non ablative lasers but did not achieve the ablation threshold. This study presents an innovative Tm:YAP laser at 1.94μm as an ablative laser. Employing high-energy, passively Q-switched pulses, at the nanosecond regime enables to reach the ablation threshold. The relatively high absorption characteristics at 1.94μm ensure the laser surpasses the lowered ablation threshold successfully. Experimental demonstrations on porcine skin using a fractional method showcased the creation of clean ablation micro columns. Micro columns with a remarkable thinness of up to 60μm was demonstrated. Deep column of up to 1.9mm was also observed. The 1.94μm Tm:YAP laser, as an innovative addition to the arsenal of ablative lasers, has the potential to revolutionize dermatological practices, providing a safe and reliable solution for skin treatments. Further refinement and development could open new avenues for enhancing patient care in dermatology.
For many years, the KGW Raman crystal has been widely used as a Raman gain medium for lasers in the visible spectrum. This paper presents recent advancements in SWIR Raman lasers, successfully implementing the KGW crystal as the gain medium in the SWIR spectral range. The KGW Raman gain coefficient is inversely proportional to the wavelength, potentially lowering the efficacy for longer wavelengths. However, the high damage threshold (exceeding 10 GW/cm2), a decent thermal conductivity coefficient as well as a highly integrated scattering cross-section effectively overcome this challenge. This paper summarizes results of different Tm based/KGW Raman lasers in external cavity configurations. Due to the KGW bi-axial properties, the Raman laser emits two discrete shifted wavelengths. The first system used a Tm:YLF pump laser and was actively Q-switched, producing short pulses of 5.4 ns. The system was the first demonstration of SWIR Raman lasing using a KGW gain medium. The second configuration used a Tm:YAP pump source and was successfully passively Q-switched using a Cr:ZnS SA resulting in output energies of 340 μJ/pulse and 450 μJ/pulse for the respective Raman wavelengths. The third Raman laser achieved a record maximum energy per pulse of 2.08 mJ for the 901 cm−1 Stokes shift utilizing a passively Q-switched Tm:YLF pump laser. To the best of our knowledge, these results present a successful demonstration of SWIR Raman conversion using KGW as the Raman gain medium, along with significant advancements in terms of conversion efficiencies and energy per pulse.
The 2 μm wavelength range provides a wide range of applications requiring short pulse durations, high peak power, and high atmospheric transmittance. Typically, Ho:YAG lasers are suitable for such applications due to their emission at 2.1 μm. Unfortunately, the existing literature reveals either short pulse durations with low energies using the cavity dumping method or high energies with longer pulse durations using the Q-switch method within this spectral range. Here, we present a novel approach that combines high energy per pulse and short pulse duration for a Ho:YAG laser, utilizing the Gain-switch method. The main advantage of the Gain- switch over Q-switch is the ability to design a short cavity, thereby achieving short pulse durations. Two different lengths of Ho:YAG crystals (20 mm or 7 mm) were tested using an actively (acousto optic modulator) or passively (Cr:ZnS) Q-switched Tm:YLF laser that was tuned to the Ho:YAG absorption peak, at 1879 nm, as a pump source. The Ho:YAG laser emitted at 2090 nm. Pulse durations of 3.35 ns with energies up to 0.7 mJ were obtained for the 20mm Ho:YAG crystal using the active or passive seed lasers. Shorter pulses of 2.3 ns with energies up to 0.35 mJ, were obtained for the 7mm Ho:YAG crystal using the passive seed laser. The reported results were limited by the output coupler’s damage threshold level. These results represent the highest achievements in terms of pulse duration and energy per pulse using the Gain-switch method.
Electro-optic active Q-switching in a Tm:YLF laser operating at 1.88 μm is demonstrated using a potassium lithium tantalate niobate (KLTN) crystal as the electro-optic medium. Upon approaching the ferroelectric phase transition temperature, KLTN manifests an exceptionally strong quadratic electro-optic effect. Operation with low driving voltages of 550V on a 3 mm wide and 2mm long crystal were demonstrated. Pulse energy of 1.4 mJ with 17 ns pulse duration was obtained at 1 kHz repetition rate with 9.5W incident pump power. A special trapezoidal configuration of the KLTN crystal was used in order to suppress the acousto-optic oscillations that are generated due to the strong electrostriction manifested in KLTN close to the phase transition temperature which lead to strong instabilities in the laser performance. New EO Q switching mechanism was implemented, in which the resonator gain rather than the loss was shifted between two levels by exploiting the different gain of the Tm:YLF crystal in the and polarizations. This obviated the need for inserting a polarizer in the laser cavity, and allowed the shortening of the laser cavity.
We demonstrate an external-cavity KGd(WO4)2 (KGW) Raman laser, pumped by an actively Q-switch Tm:YLF MOPA. The fundamental spectral line emitting at 1881 nm allowed the KGW bi-axial crystal to lase at two separate output spectral lines, 2198 and 2265 nm, depending on the seed polarization axis relative to the KGW's axis. The Tm:YLF seed was amplified using a double-pass Tm:YLF crystal based MOPA setup. After amplification, the seed achieved an output power of 9.15 W, and an energy pulse of 4.57 mJ, a pulse duration of 43 ns at a repetition rate of 2 kHz. The max output average power achieved for the 2265 nm was 1.85 W, with a pulse energy of 0.923 mJ at a repetition rate of 2 kHz implying a conversion efficiency of ~20.5%. We noticed a very low conversion efficiency of the shorter KGW spectral shift (at 2198 nm). The reason for this efficiency drop was validated to be the 2nd stokes forming and thus consuming the 1st stokes energy. In favor of the KGW inherent properties and according to the aforementioned results, this crystal appears to be suitable for power scaling as well as for improvement of the Raman conversion efficiency in this spectral range. The KGW crystal is well known for its use in shorter spectral wavelengths. To the best of our knowledge, it is the highest average power achieved by lasing in the 2 μm region using SRS with KGW.
Tunable and milli-Joule level pulsed Tm based laser, are demonstrated in this paper. The spectral bandwidth was narrowed down to 0.15 nm FWHM. For the Actively Q-switched Tm:YLF laser, we achieved 33 nm of tunability range between 1873 nm and 1906 nm, using a pair of YAG Etalons. Using the same tunability technic for the Passively Qswitched Tm:YAP laser, we achieved 11 nm of tunability between 1930-1941 nm. The Tm:YLF laser was actively Qswitched using an acousto-optic modulator, while achieving mJ level pulse energy along the whole tuning range at a repetition rate of 1 kHz. Up to 1.97 mJ of energy per pulse was achieved at a pulse duration of 37 ns at a wavelength of 1879 nm, corresponding to a peak-power of 53.2 kW and at a slope efficiency of 36 %. The Tm:YAP laser was Passively Q-switched using Cr:ZnS saturable absorber (SA) as modulator, achieving mJ level pulse energy along the whole tuning range. Up to 1.2 mJ of energy per pulse was achieved at a pulse duration of 24 ns at a wavelength of 1935 nm, corresponding to a peak-power of 50 kW. The combination of both high energy pulsed lasing and spectral tunability, while maintaining narrow bandwidth across the whole tunability range, enhances the laser abilities, which could enable new applications in the sensing, medical and material processing fields. Which in the case of the of the Passively Qswitched Tm laser has a major advantage in terms of foot print.
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