| 3 μm lasers are widely used in biomedicine,nonlinear optics,hazardous chemical detection,atmospheric remote sensing,and pollution monitoring due to its wavelength at the absorption peak of water and hydroxyl.The high repetition frequency,high peak power and short nanosecond pulse width of 3 μm lasers are necessary,when they are used as a mid-infrared parametric pump source,for the ablation of tooth hard tissue.A high repetition frequency and high peak power nanosecond pulsed laser can effectively improve optic-optic conversion efficiency,reduce thermal damage during the ablation of tooth hard tissue,and enhance dentin shear bond strength of universal adhesives.3 μm waveband lasers with high repetition frequency are usually pumped by LD,which is expensive and completely dependent on imports.However,3 μm waveband lasers pumped by xenon flash-lamp are unfavorable to the popularization of this band laser due to serious thermal effect and low repetition frequency.Nanosecond pulses and high peak power at a waveband of 3 μm can be accomplished via various Q-switching techniques.Electro-optical Q-switched technology has achieved a large energy output in the 3 μm waveband,but owing to thermal depolarization,it cannot achieve high repetition frequency operation.Acousto-opticl Q-switched technology can achieve high repetition frequency operation but cannot achieve large energy output.At present,passively Q-switched lacks saturable absorber with high damage threshold,which cannot achieve high repetition frequency and high pulse energy.At the same time,there are different opinions about the influence of long pulse erbium laser ablation on the bonding strength,and the morphology and bonding strength of high repetition frequency nanosecond short pulse erbium laser ablation dentin are in urgent need of verification.In order to solve the above problems,this paper mainly develops Fe:ZnSe passively Q-switched Er,Cr:YSGG laser pumped by xenon flash-lamp with high repetition frequency and peak power.Then dentin morphology was observed and bonding strength was tested after nanosecond-and microsecond-pulse Er,Cr:YSGG lasers ablation,which proved the superiority of nanosecond pulse laser over microsecond pulse laser.The main research results of this study are as follows:In terms of theoretical research:1,The influence of initial transmittance of saturable absorber and output mirror reflectance on energy density in resonant cavity is theoretically calculated,and the value range of initial transmittance of saturable absorber and output mirror reflectance are determined.The output energy density and pulse width curves of passively Q-switched crystal with different initial transmittance are obtained.2,The measurement and compensation method of thermal lens focal length at high repetition frequency were theoretically analyzed,and the resonant cavity structure matching the focal length of thermal lens was designed to compensate the thermal lens effect effectively.The distribution of beam diameter in the resonant cavity is theoretically calculated to provide guidance for the placement of passive Q-switched crystal.In terms of experimental research:1,This paper carried out the high repetition frequency flash lamp pumped Er,Cr:YSGG laser study,thermal focal length was measured at different pump power.An optimized cavity structure to match the thermal focal length was designed,and improveed the laser energy.100 Hz and 60 Hz frequency 2.794 μm wavelength laser with high energy output operation at free running was realized,and the output energy are 62.3 mJ and 202 mJ respectively.The output energy of concave-convex cavity areincreased by 2.9 times and 1.98 times respectively compared with the plane-parallel cavity when the same energy is injected.2,This new structure protects the saturable absorber from damage caused by the high peak power density with a small beam size.The concave-convex cavity expands the beam diameter from 0.93 mm to 1.57 mm compared to the plane-parallel cavity,Therefore,a high-repetition-frequency and high-energy Cr,Er:YSGG laser was realized.Fe:ZnSe with an initial transmittance of 93.6%was used to carry out passively Q-switched experiment,and the influence of the position of passively Q-switched components on the output energy was investigated.When Q-switched operating at 100 Hz,single pulse with 4.3 mJ pulse energy and 107.6 ns pulse width was obtained.Multi-pulse operation was also obtained,yielding 20 mJ of output energy over five pulses.When Q-switched operating at 60 Hz,the device produced single pulse of 4 mJ energy with 110.4 ns pulse width and multi-pulses(13 pulses)with 60.3 mJ total output energy.3,The dentin ablation morphology of Er,Cr:YSGG laser with different pulse width at 100 Hz repetition rate was compared.The dentin sample irradiated with a repetition rate of 100 Hz with a100 ns pulse width and an energy 3 mJ laser shows a well-organized,very clean dentin structure,presenting a series of closely arranged and smooth "craters" in the dental ablation zone.No carbonization,melting,or debris is observed on the surface,and the lower dentinal tubules are completely open.However,after 3 mJ energy 150 μs pulse width laser ablation of dentin,the ablation depth became shallow,the gully edge was irregular,and the dentinal tubules were partially open,indicating that melting had occurred.When the laser energy was increased to 20 mJ,the morphology of the gully edge included many irregular cracks,the surface of the gully was scorched yellow(visible with to the naked eye),and the dentinal tubule was partially sealed.4,Nanosecond and microsecond pulses Er,Cr:YSGG laser with 100 Hz repetition frequency were used to prepare dentin surfaces,and the resulting SBS with universal adhesive was measured.The resulting SBS of nanosecond-pulse laser ablation was significantly higher than when using a long pulse width laser,both with and without acid-etching.These results show that nanosecond-pulse laser ablation has potential application value for preparing dentin without using an acid-etching agent and for improving the bond strength when used in combination with acid etching.The microsecond-long pulse laser requires acid-etching,and the energy should not be too large during any ablation process without water mist cooling. |
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