Theoretical And Experimental Studies On Nd:YAG Ceramic Lasers And Investigations On Light Diffusion In TiO₂ Random Media

There are two parts in this dissertation.One is the theoretical and experimental studies on Nd:YAG ceramic lasers.The other is the investigations on the light diffusion in TiO₂ random media.Neodymium doped yttrium aluminum garnet(Nd:YAG) single crystal has been the most widely used solid-state laser material.But its fabrication requires expensive equipment and crucible material.Neodymium doped yttrium aluminum garnet(YAG) transparent ceramics are nowadays attracting a great interest as a laser host material because of its advantages over the traditionally used Nd:YAG crystals.These advantages are the excellent thermal properties,the lower manufacturing costs,the possibility of high neodymium concentrations without any decrease in the optical quality of the gain medium,no growth limitation of shape and size,ease of power scaling,and the possibility of direct composite fabrication.Because of these advantages,excellent quality Nd:YAG ceramic laser materials are good alternative to the widely used Nd:YAG single crystals.Many scientists are engaged in the investigations of ceramic laser materials and ceramic lasers in recent years,and have made many achievemants.Scientists from Japan act as the leading role in the regime of ceramic growth and research on the ceramic lasers.Scientists from Russia,America,Germany,Ramania,Spain etc.are taking part actively in the field of the ceramic lasers.In China,the research groups from Shandong University,Shanghai Institute of Optics and Fine Mechanics,and Tsinghua University work at the research of the ceramic lasers.Rate equations are efficient tools describing the operation of lasers.The rate equations considering the spatial distributions of the intracavity photon density and the population inversion density for the four-level Q-switched lasers have been analyzed and widely cited.So far,there are no rate equations for a quasi-three level actively Q-switched lasers considering the influence of the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density.Photon localization is proposed as the analog of electron localization.It refers to an inhibition of wave transport in scattering media due to the interference of multiple scattered waves.The time-resolved measurement of light diffusion in random media is an effective method to investigate photon localization.In this dissertation,we have studied Nd:YAG ceramic lasers theoretically and experimentally.By using ultrashort pulses,we have studied the time-resolved transmission properties of TiO₂ random media.The main content of this dissertation is as follows:1.Diode-pumped continuous wave,actively Q-switched and passively Q-switched ceramic lasers operating at 1064 nm are investigated.Comparative studies of Nd:YAG ceramic lasers and Nd:YAG single crystal lasers are performed.The laser performance of the Nd:YAG ceramic is found to be a little better than that of Nd:YAG single crystal.2.The characteristics of the 946 nm actively Q-switched ceramic lasers are investigated.Using a fiber-coupled laser diode with a fiber diameter of 600μm as the pump source,an average output power of 2.6 W at a repetition rate of 50 kHz is obtained when the incident pump power is 17.4 W.3.The rate equations for the quasi-three-level actively Q-switched lasers are obtained by considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density.These rate equations are normalized by introducing some synthetic parameters.By solving the normalized rate equation numerically,a group of general curves are generated.These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of any quasi-three-level actively Q-switched laser.The laser pulse characteristics of the 946 nm actively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.4.By introducing some synthetic parameters into the rate equations considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density,the normalized rate equation for the quasi-three-level passively Q-switched lasers is obtained.By solving the normalized rate equation numerically,a group of general curves are generated. These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of many quasi-three-level passively Q-switched lasers.The laser pulse characteristics of the 946 nm passively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.5.Time-resolved transmission properties of 532 nm ultrashort pulse(60 ps pulse width,10 Hz repetition rate) in TiO₂ suspended solutions are investigated.A streak camera is used to record the transmitted signals.The diffusion constant is obtained by theoretical fitting,and the relation between the diffusion constant and the particle density of TiO₂ in the suspended solution is studied.The diffusion constant decreases with increasing particle density.6.Time-resolved transmission properties of 800 nm ultrashort pulse(10 ps pulse width,100 Hz repetition rate) in TiO₂ powders are investigated.A digital storage oscilloscope is used to record the transmitted signal.When the particle density is 1.36×10¹⁵ cm^-3,the experimental result shows a little deviation from the diffusion theory,which may be the signature of weak localization in the random media.The main innovations of this dissertation are as follows:1.The diode-pumped actively Q-switched Nd:YAG ceramic laser operating at 946 nm is obtained for the first time.Using a fiber-coupled laser diode with a fiber diameter of 600μm as the pump source,an average output power of 2.6 W at a repetition rate of 50 kHz is obtained when the incident pump power is 17.4 W.2.The rate equations for the quasi-three-level actively Q-switched lasers are obtained for the first time by considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density.These rate equations are normalized by introducing some synthetic parameters.By solving the normalized rate equation numerically,a group of general curves are generated.These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of any quasi- three-level actively Q-switched laser.The laser pulse characteristics of the 946 nm actively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.3.By introducing some synthetic parameters into the rate equations considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density,the normalized rate equation for the quasi-three-level passively Q-switched lasers is obtained for the first time. By solving the normalized rate equation numerically,a group of general curves are generated,These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of many quasi-three-level passively Q-switched lasers.The laser pulse characteristics of the 946 nm passively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.4.Time-resolved transmission properties of ultrashort pulse in TiO₂ random media are studied.The relation between the diffusion constant and partical density in TiO₂ random media is investigated for the first time.When using the TiO₂ powders as the sample with a particle density of 1.36×10¹⁵ cm^-3,the experimental result shows a little deviation from the diffusion theory,which may be the signature of weak localization in the random media.