To measure the effect of laser fluence and scanning velocity on ablation efficiency of enamel and dentin.Two extracted human incisors and two molars were cut transversely along the axial plane with a diamond saw to obtain dentin and enamel slices with thickness of about 1 mm. Samples were fixed on a motorized translation stage, the linear reciprocating movement in the plane perpendicular to the direction of laser incident was programmed by the controller, and the laser focused on the tooth surface, then 36 ablation lines on enamel and 48 ablation lines on dentin were produced. A femtosecond laser system with wavelength of 800 nm, pulse width 30 fs, repetition frequency 1000 Hz was used, and the diameter of the focused spot was approximately 25 µm. A group of different fluence (1.33, 1.77, 2.21, 4.42, 8.85, 17.69 J/cm(2) for enamel and 0.44, 0.66, 0.88, 1.33, 1.77, 2.21, 4.42, 6.63 J/cm(2) for dentin) and two scanning velocity (10 mm/s and 20 mm/s) were tested. Confocal laser scanning microscope was used to measure the ablation volume.Ablation efficiency for enamel and dentin was then calculated.Under the fluence of 8.85 J/cm(2) there was the highest ablation efficiency for enamel, 18.703×10(-3) mm(3)/J (20 mm/s), and the highest ablation efficiency for dentin was found under the fluence of 2.21 J/cm(2), ie.223.458×10(-3) mm(3)/J (20 mm/s).Fluence and scanning speed of this femtosecond laser can affect ablation efficiency for both enamel and dentin, and this suggests that with appropriate choice of fluence and scanning speed we can improve the ablation efficiency for enamel and dentin.
A joule-level Nd: YAG nanosecond laser of high repetition frequency and high beam quality is developed for Thomson scattering diagnosis. The laser is designed as a master oscillator power-amplifier system mainly including single longitudinal mode seed, pre-amplifier unit and energy extraction unit. The single-longitudinal-mode Q-switched laser of a high stability is taken as the seed laser of output pulse at J level. The pre-amplifier unit amplifies the J-level pulse laser beam into hundreds of mJ level. In order to obtain the high-quality laser beam output, phase conjugation is adopted to compensate for the laser beam distortion. The ultra-filtered FC-770 is taken as an SBS gain medium of 0.0011 cm-1 absorption coefficient, 197.9 GW/cm2 optical breakdown threshold and 3.5 cm/GW gain coefficient. The double-pass amplification of SBS phase conjugation could realize a real-time repair towards the non-uniformity, deformation and wavefront aberration caused by thermal distortion of the optical components and the laser amplifier to achieve the uniform amplified beam output of high quality close to the diffraction limit. In the energy extraction unit, the amplifier of large-diameter slab is used for energy amplification. The size of the slab is 7 mm 35 mm138.2 mm of 56 cutting angle and 0.6% Nd3+ doping concentration. The slab is plated by a layer of SiO2 against light leak. Horizontal pumping mode is adopted. And the slow axis of the laser diode is almost the same as the length of the slat and the direction of laser transmission. The single-plane array is composed of 8 groups of vertical stacks and each group consists of 12 laser diode bars of power 200 W. At 200 Hz repetition frequency, 250 s pump pulse width and 140 A pump current, the up to 2.3 J stored energy can be achieved The energy extraction unit achieves high gain amplification and finally outputs high-quality laser beam. Under the condition of 200 Hz high repetition frequency and 8.23 J single pulse energy injected by the single longitudinal mode seed, 1.85 J output energy is gained. The energy extract efficiency of the laser system is 52.46%. The output laser possesses a pulse width of 5.36 ns, a far field beam spot 1.72 times the diffraction-limited value, and 1.3% energy stability (RMS).
According to the principle of longitudinal-mode-selecting by etalon in pulsed single longitudinal laser, the influences of thickness accuracy and position accuracy of various F-P etalons on the performance of longitudinal mode selecting are analyzed quantitatively. In order to compensate the thickness deviation of etalon, the angle detuning of various etalons are calculated. Then the effects of the incident angle deviation on transmittance curve of F-P etalon are investigated. These are important for setting the mechanical structure design, the tolerance of devices, and the adjustment accuracy of the pulsed single longitudinal laser. In the linear-cavity laser with an F-P etalon for mode-selecting, a near-diffraction-limited single longitude pulsed output is obtained. The maximum single pulse output energy is 8.41 J and the pulse width is 32 ns.
With the rapid development of space technology, human activities into space are increasing, thereby producing lots of space debris. And the space debris impact is the major cause for the mechanical damage to the space crafts and the main factor affecting the service life; it even endangers the life safety of the astronauts working outside the spacecraft and pose a threat to the astronomical observation and studies. Thus, the monitoring and early warning of space debris are gradually attracting wide attention. Obviously, laser detection as a good-directivity and strong anti-jamming active detecting means has a unique advantage in terms of a round-the-clock detection. Therefore, the developing of debris-detecting laser beam source becomes the most direct and effective means for increasing the space debris detection accuracy. The laser detecting ability is restricted by the laser beam quality, the pulse energy and the repetition frequency at the same time. The beam quality could affect the ability to detect and recognize space target. The bigger the laser pulse energy, the higher the repetition frequency and the smaller the detectable debris, the stronger the detecting ability will be. A good detection effect could be achieved at 80-100 Hz laser pulse repetition frequency. A further increase of the repetition frequency will greatly increase the difficulty and cost accordingly but the improvement of the detection performance is not obvious at all. Thus, repetition frequency around 100 Hz becomes the best choice for laser space debris detection. Based on the laser diode side-pumped rod-shaped amplifier, a high-repetition-frequency and high-beam-quality of joule level Nd:YAG nanosecond laser for space debris detection is developed in this work. The laser adopts MOPA structure, mainly including single longitudinal mode, pre-amplifier unit, SBS phase-conjugate beam control unit and energy extraction unit. In the energy extraction unit, beam splitting-amplifying-combining is adopted for reducing the thermal effect on beam quality by reducing the working current of the amplifier. Under the condition of 100 Hz high repetition frequency and 10.73 J single pulse energy injected by the single longitudinal mode seed, 3.31 J output energy is gained. The output laser beam has a 4.58 ns pulse width, far field beam spot of 2.12 times the value of the diffraction limit, and 0.87% energy stability (RMS).
1.3 μm Nd laser has significant practical applications in various fields, such as fiber communication, medical treatment, frequency conversion, and scientific research, etc. Many applications of a 1.3 μm laser, especially like frequency conversion, benefit much from a short pulse width with high peak power. In the paper, an electro-optical cavity dumping Nd:YVO<sub>4</sub> laser at 1342 nm wavelength has been studied theoretically and experimentally. The pulse width for an electro-optical cavity dumping laser is determined by the optical length of the cavity. A narrower pulse width can be obtained by reducing the length of the cavity and the round trip time of the laser in the cavity. However, when the round trip time in the cavity approaches to the falling edge time of the electro-optical switch, shortening the length of the cavity will not get a narrower pulse width, and the falling edge time of the electro-optical switch will influence the laser pulse width. The temporal characteristics of the laser pulse are simulated when the falling edge time of the electro-optical switch is close to the round trip time in the cavity. Influences of the falling edge time of the electro-optical switch on the laser pulse duration are analyzed theoretically. The modified rate equation is used to study the relationship between the falling edge time and the laser pulse width. We demonstrate an electro-optical cavity dumping Nd:YVO<sub>4</sub> laser. The 0.3 at.% Nd:YVO<sub>4</sub> placed in a short Plano-concave cavity is in-band pumped by an 880 nm quasi-continuous-wave diode. A fiber-coupled diode laser module (NA=0.22) with a power of 30 W was used. A LiNbO<sub>3</sub> electro-optical switch was employed for the cavity-dumping. The 1342 nm cavity-dumping laser operates at a repetition rate of 1kHz, and a single-pulse energy of 0.21 mJ is obtained with a pulse width of 2.8 ns. Near-diffraction-limited beam quality with an M<sup>2</sup> value of < l.2 is achieved. The setup provides efficient second harmonic generation at 671 nm using a MgO:PPLN crystal, and the pulse width is 1.8 ns. To the best of our knowledge, this is the shortest pulse duration obtained from 1.3μm actively Q-switched Nd-doped laser.
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