| The wavelength of the mid-infrared 3 ?m laser is in the absorption peak of water and the fingerprint region of the infrared spectrum.It has been widely used in the fields of biological medicine,scientific research and military affairs,etc.3 ?m laser has become a research hotspot in the field of laser.Q-switch is one of the main technologies to obtain high-peak-power laser pulse with narrow pulse width.Compared with other 3?m laser crystals,the Er:YSGG laser crystal has relatively longer upper-level life time and lower threshold.So the Er:YSGG is a efficient laser crystal to generate 2.79 ?m laser.At present,there are two main problems with high-repetition-rate acousto-optic Q-switched 2.79 ?m lasers.One is the optical and physical properties of 2.79 ?m laser crystal limit the parameters of 2.79 ?m lasers.The thermal lens effect and thermally induced birefringence effects seriously affect beam quality and pulse energy;the second one is the lack of acousto-optic Q-switch devices with low insertion loss,high diffraction efficiency and high damage threshold.The safety of free-running and Q-switch 3 ?m lasers on human teeth needs to be further verified experimentally aslo.To solve the problems mentioned above,a high-repetition-rate acousto-optic Q-switch Er:YSGG laser has been developed and carried out experiments of laser ablation of dentin of human teeth.The main research contents of this thesis are as follows:1.In the LD side-pumped Er:YSGG acousto-optic Q-switched laser.The drop of pulse energy caused by thermal lens was compensated effectively by design the laser cavity.The acousto-optic Q-switch made of TeO2 crystal can modulate the loss of laser cavity periodically to obtain the Q-switch laser output.As a result,a Q-switched 2.79?m laser with a repetition rate of 100-300 Hz is obtained.When a 100 Hz repetition rate,a 1 mJ pulse energy and a 76 ns pulse duration are employed,the corresponding peak power is 13.2 kW.2.The ablation of teeth by laser was studied theoretically.The Abaqus software was used to simulate the temperature distribution of ablation of teeth by pulsed laser.It was found that compared with the long pulse-width laser,the thermal diffusion area during short pulse-width laser ablation is smaller.The pulse energy of the laser should be controlled to reduce the stress generated during ablation.3.The influence of pulse-width on erbium free-running laser ablation of hard dental tissue were studied.Results shows that with the increase of pulse-width,the ablation threshold and ablation depth showed a trend of increasing gradually.4.The influence of water mist on Q-switched erbium laser ablation of dental tissue is investigated.The result of the experiment shows that the increase of the flow of water mist can rise the Q-switched laser ablation threshold and the damage threshold to dentin5.The characteristics of dentin ablation with a high-repetition-rate Er:YSGG laser are investigated.Compared with traditional Q-switched Er:YSGG lasers,high-repetition-rate Q-switched Er:YSGG lasers improve the ablation efficiency without causing thermal and mechanical damage.The temperature of pulp cavity in ablating dental hard tissues was kept below 42? when using Q-switched laser. |
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