Research On Short Pulse Laser Generation Technology And Its Application

The laser pulses with picosecond or femtosecond duration can be obtained by Q-switching or mode-locking techniques.The picosecond laser pulses can be directly output by micro-cavity Q-switching technology,and then the femtosecond laser pulses can be obtained by nonlinear spectral broadening and linear chirp compensation.The picosecond and femtosecond laser pulses with lower noise can be obtained by using mode-locking technology.The extremely short pulse duration can obtain the extremely high time resolution and the extremely high peak power,which has great application value in the fields of ultra-fast detection,laser manufacturing,photoelectric detection,etc.In this dissertation,the method of preparing graphene by femtosecond laser and the output characteristics of a passively Q-switched laser based on graphene were researched.The generation of microwave frequency high energy subnanosecond pulse and its application in the light detection and ranging-radio detection and ranging(LIDAR-RADAR)system were studied.The passive mode-locked ytterbium-doped potassium gadolinium tungstate(Yb³⁺:KGd(WO₄)₂,Yb:KGW)crystal laser with semiconductor saturable absorbent mirror and its application in the pump-probe thermal reflection system for testing the thermo-physical properties of thin film materials were studied.The main work content and the innovation achievements are as follows:(1)The technique of graphene exfoliated by femtosecond laser was studied,and a passive Q-switched laser was built based on the prepared graphene.The size of the graphene flakes was well regulated by controlling the laser peak-power density.The image of transmission electron microscope shows that the graphene has good morphology and crystal phase.The hexagonal lattice structure of the graphene carbon ring is clearly visible in the high-resolution transmission electron microscopy image.In addition,the graphene dispersed in deionized water was used as a saturated absorber in the passively Q-switched laser.The maximum pulse energy of 262.35 n J and the narrowest pulse width of 131.6 ns were obtained.(2)The generation of microwave frequency high energy subnanosecond pulse and its application in LIDAR-RADAR system were studied.A neodymium-doped yttrium aluminum garnet and chromium-doped yttrium aluminum garnet bonded crystals(Nd:YAG/Cr⁴⁺:YAG)short cavity passively Q-switched laser with the pulse energy of 0.37 mJ and the pulse width of 760 ps was constructed.After two-stage dual-pass amplification,the pulse energy was increased to 178.1 mJ.A 532 nm laser pulse with the energy of 87.6 mJ was obtained based on the potassium titanium oxide phosphate(KTiOP₄,KTP)crystal frequency doubling technique.The frequency doubling efficiency was 49.2%and the energy instability(Root mean square,RMS)of 20000 consecutive 532 nm laser pulses was 1.276%.On the basis of the above work,the pulse output of microwave frequency laser with pulse frequency of 500 MHz and pulse energy of 26.3 mJ was obtained by the time domain control of the external resonator.A LIDAR-RADAR system,which combines the microwave frequency laser source with the streak tube imaging,has been successfully developed,and the clear 3-D intensity image,3-D range image,and 4-D point cloud image of the target has been obtained in the underwater target detection experiments.By using microwave frequency laser source,the signal-to-clutter ratio of echo signal received by the LIDAR-RADAR system has been greatly improved,and the detection distance has been significantly increased.The spatial resolution of the LIDAR-RADAR system is less than 9 mm across 20 m in the clear water environment.(3)A diode-pumped Yb:KGW solid-state mode-locked laser and its application in a dual-wavelength femtosecond laser pump-probe thermal reflection system were studied.An experimental result obtained the pulse width of 290 fs and the output power of 6.12 W by using a SESAM with the modulation depth of 1.2%and two GTI reflectors for compensating the group delay dispersion.The dissipative soliton operation of Yb:KGW crystal mode-locked laser in the all-positive-dispersion region was studied.Stable mode-locked pulses with single pulse energy of 39.8 n J,maximum average power of 2.07 W and pulse width of 4.317 ps are obtained.The pulse width can be compressed to 300 fs by the grating pair pulse compressor.The experiment results show that the dissipative soliton has larger pulse energy,stronger soliton stability and great chirp.A dual-wavelength femtosecond laser pumping system for detecting thermal reflection was built by using the Yb:KGW mode-locked laser.By measuring the thermal conductivity of monocrystalline silicon and comparing with the standard value,the results show that the system has higher testing accuracy.