| Atoms,molecules,or ions are arranged in periodic order in three dimensions to form crystals.It has been proved that the crystal is a kind of medium with much importance to realize the mutual conversion of force,heat,sound,light,electricity and magnetism.The crystal plays an important role in promoting the progress of the scientific and technological revolution.Crystals can be divided into many types according to their functions,such as nonlinear optical crystals,magneto-optic crystals,acousto-optic crystals,piezoelectric crystals and so on.The yield of natural crystal is much lower,and the types,size and quality of crystals cannot meet the needs of science and technology,while artificial crystal has shown well-developed superiority.Since time immemorial,people have been engaged in crystal growth activities such as boiling sea water for salt,refining and so on.After nearly a thousand years of development,crystallography has gradually developed into a discipline,which plays a significant role in crystal growth,crystal characterization and crystal applications.A wide variety of artificial crystals have greatly promoted the development of laser,medical,electronic,military and other fields of technology.Acousto-optic modulation is an effective method to achieve high peak power laser.With the development of laser technology,especially of the fiber laser,efficient acousto-optic crystals and devices have become a hot research field.TiTe3O8(TTO)crystal is a new acousto-optic crystal with excellent performance,exhibiting large sound velocity and large figure of merit M2.TTO crystal belongs to cubic crystal system showing optical isotropy,which indicates that TTO crystal is an ideal material for the fabrication of fiber coupled acousto-optic modulator with high polarization extinction ratio.According to the analysis above,TTO crystal is chosen as the research object in this paper.It is focused on the optimization of parameters for the growth of TTO single crystal,the design and fabrication of the fiber coupled acousto-optic Q-switching modulator.Beside the acousto-optic characteristics of TTO crystals,rare earth ion doping was also carried out to explore the multifunctional applications of TTO crystals.This paper is divided into five chapters,the first one is the introduction,the fifth one is the conclusions.The main research contents from second to fourth chapter are as follows:Ⅰ.Optimized growth of large TTO single crystals and measurement of some physical propertiesAccording to the quality and size requirements of the TTO single crystals,optimized crystal growth parameters are considered from 3 aspects:the cooling rate,the rotation rate and the crystal seed direction.The large TTO crystal with weight of 252 g was successfully obtained after growth parameters optimization,which provides a material basis for the study of physical properties and devices.In the initial,TTO crystals grown using the flux method have many macroscopic defects.TTO crystal weights up to 252 g with high quality was obtained by optimizing the growth parameters,such as slow down the cooling rate and the rotation speed,changed the[100]-oriended to[111]-oriended seed crystal,and so on.Some physical properties of the TTO crystal were characterized.The Mohs hardness of TTO crystal was 5.7 and the laser damage threshold was 226.93MW·cm-2(@1064 nm,10 ns).Ⅱ.Fiber acousto-optic modulator based on TTO crystalIn order to meet the urgent need of polarization-maintaining acousto-optic modulators in fiber lasers,a fiber coupled acousto-optic Q-switching modulator with polarization maintaining ability was designed and fabricated by taking advantage of the isotropic optical properties of TTO crystal.The device work at 1064 nm,an insertion loss of 1.8 dB,a rise time of 36 ns,an on-off extinction ratio of 58 dB,and a polarization extinction ratio of 19dB are obtained.The device has been successfully applied in ytterbium doped fiber laser.Stable pulse trains with a central wavelength of 1060nm are obtained,and the minimum pulse width is 44ns.A single-mode optical fiber acousto-optic modulator is fabricated by optimization of the electrode size.The insertion loss decreased from 1.8 dB to 1.5 dB,and the on-off extinction ratio increased from 58 dB to 60 dB.The preliminary results show that the performance of TTO acousto-optic modulators could satisfy the technique requirements of the commercial devices with good prospect for practical application.Ⅲ.Growth and application of ion-doped TTO crystalsTransition metal ions or rare earth ions show unique properties.A piece of Cr4+:TTO single crystal of centimetre-level was grown by flux method,such a crystal is expected to realize the combination of active and passive Q-switch.The basic physical properties of Cr4+:TTO crystal were characterized.The absorption spectrum shows that the crystal has a characteristic absorption peak of Cr4+ in the band range of 1060 nm.A passive Q-switching test at 1064 nm was carried out using the Cr4+:TTO crystal as the saturable absorber.A passive q-switched pulse train with pulse widths of 1 μs at the repetition rate of 67 kHz was observed.The maximum output power is 50 mW when the pump power is 1.194 W.However the optimal doping concentration of Cr4+ needs to be explored.Besides,pieces of Nd3+:TTO crystals were grown by flux method to explore the application in laser field. |
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