China
Africa
Explore chapters and articles related to this topic
Basic Principles
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
In general, high-power lasers do not produce a single transverse mode, due to thermal effects in the laser gain medium. Heating gives rise to a non-uniform temperature profile and, hence, thermal lensing. Conventional lenses are often placed within the laser resonator to compensate for this lensing effect but residual effects remain, which lead to the generation of higher order modes. This is particularly an issue for lamp-pumped solid-state lasers, where only a small percentage of the broadband pump light has the correct wavelength to pump the laser, with the remainder being converted into heat. This means that a typical Nd:YAG welding laser will have a highly multi-mode beam with a beam profile which is roughly ‘top-hat’. Spatial filtering can be used to reduce the number of modes; however, this also reduces the average power. Narrow-band pumping can significantly increase efficiency and, hence, reduce heating effects, and so high-power laser diode arrays are being increasingly used as pump sources for Nd:YAG lasers. The capital cost of these arrays is significantly higher than flash lamps; however, they do have advantages in terms of maintenance cost, with typical lifetimes of 10 000 h compared with 1000 h for flash lamps. With gas lasers, thermal lensing effects are significantly reduced since the dependence of refractive index on temperature is several orders of magnitude lower than with solid-state lasers; also, novel resonator designs such as the hybrid unstable resonator [3] have been used to remove heat more efficiently.
Thermal Issues
Published in Mark Steven Csele, Laser Modeling, 2017
Heat produces two effects in solid-state lasers that are detrimental to laser action. The first effect, thermal population of low-lying LLLs resulting in re-absorption loss, can occur due to heat from quantum defects in the medium itself. The second effect, which can limit the ultimate power of a high-power laser, is thermal lensing of the amplifier medium itself.3 Thermal lensing is an effect by which a temperature gradient within a solid-state laser material causes it to optically distort and so act as a lens. In the case of a solid-state laser, several effects are possible depending on the geometry of the laser. This effect is most prominent in large solid-state lasers that are side-pumped and in which the rod is cooled from the outside surface. Heat, a result of quantum defect, builds from the center of the rod—the intensity is usually higher at the center of the rod, regardless, since the mode has highest intensity there—and is cooled from the outside of the rod, so a thermal gradient develops between the hotter central and cooler peripheral regions of the amplifier, resulting in both thermal stresses and a change of refractive index of the amplifier material usually resulting in the formation of a positive lens (i.e. as if the flat ends of the amplifier were actually convex). Thus changes to the optical cavity occur, as outlined in Figure 5.23, which ultimately reduce the output power.
Renovating electrical power-to-TEM00 mode laser power conversion efficiency with four-lamp/four-rod pumping scheme
Published in Journal of Modern Optics, 2021
Miguel Catela, Dawei Liang, Cláudia R. Vistas, Dário Garcia, Bruno D. Tibúrcio, Hugo Costa, Joana Almeida
Analogously to the proposed design, the length between the HR1064 nm mirror and the left AR1064 nm end face of the laser rod (L1) of the state-of-the-art model was also optimized in order to achieve the best overlap between the pump volume and the fundamental laser mode volume, whereas L2 was fixed at 60.0 mm. L1 is a key parameter for the efficient extraction of TEM00 mode laser power due to the power-dependent thermal lensing of the laser material. As L1 increases, high-order modes are suppressed, and the fundamental mode size grows up. Therefore, to obtain efficient extraction of TEM00 mode laser power, the laser should operate close to the edge of the optically stable region, which means that L1 must approach the thermal focal length (f) of the laser rod end. For the 4 mm diameter laser rod of the SL902 Spectron single lamp model, maximum TEM00 mode laser extraction was numerically found at L1 = 178.0 mm, with the radius of curvature of −0.3 m for the HR mirror (RoC1 = −0.3 m) and 100.0 m for the PR mirror (RoC2 = 100.0 m) (Figure 6(a)). Whereas, for the 2.3 mm diameter rods of the four-lamp/four-rod scheme, the maximum TEM00 mode laser power was found at L1 = 286.0 mm, with RoC1 = 0.8 m and RoC2 = 100.0 m (Figure 6(b)). Despite the differences of about 8% and 10%, between the numerical values (L1) and the analytical values (f) of the state-of-the-art model and the proposed design, respectively, both analyses demonstrate the significant increase of the thermal focal length with the four-lamp/four-rod pumping scheme. Therefore, the proposed approach ensures a significant reduction of the thermal lens effects and, consequently, an improved TEM00 mode laser performance.
Flashlamp-pumped Nd:YAG laser with higher pulse energy using TiO2 nanofluid as coolant
Published in Journal of Modern Optics, 2019
D. Razzaghi, M. Arshadi Pirlar, M. Sasani Ghamsari
Solid state laser performance is adversely affected by the thermally induced losses such as thermal lensing and birefringences (13,14). Usually to compensate the thermal effects an appropriate cooling system is used in flashlamped-pumped Nd:YAG lasers. Also it is well known that nanofluids with enhanced thermo physical properties such as thermal conductivity, thermal diffusivity, viscosity and convective heat transfer coefficients can be employed as coolant like oil or water (15,16). The nanofluids clearly exhibit enhanced thermal conductivity, which goes up with increasing volumetric fraction of nanoparticles. The thermal conductivity of nanofluids is effectively influenced by several important factors such as particle size and shapes, clustering of particles, temperature of the fluid, and dissociation of surfactant (17,18,19). It is also proved that TiO nanofluid exhibits good UV absorption and violet-blue radiation emission which is potentially useful to increase the efficiency and manage the UV degradation (20).On the other side, TiO2 nanofluid absorb more pump radiation in the visible region in comparision with pure deionized water. This potentially degrades the pump efficiency and we will discuss it later.In our opinion, there are two main reasons for increasing the laser pulse energy when nanofluid is used. First reason is the increased heat conductivity of the coolant using TiO nanoparticles which cause a rapid heat transfer from lamp and rod (20). In comparison with pure deionized water, oleic acid + TiO nanofluid has better heat removal performance.It can decrease the thermal lensing as well as the thermally induced birefringence losses in the cavity so that higher output energy is expected using nanofluid as coolant.