Femtosecond Laser Inscription Optical Waveguide In Crystal:Fabrication,Properties And Application Research

The optical waveguide can effectively integrating the optical circuit.It is a basic component in integrated photonic systems.Waveguides fabricated in optical materials with various properties can be used as photonic devices for realizing different functions,such as optical amplifier,waveguide laser,beam splitter,optical modulator,optical switch,frequency converter,etc.Therefore,optical waveguides play an important role in integration optics and modern optical communications.Optical crystals are common matrix materials for waveguide structure fabricating.They have excellent physical,chemical and optical properties.In particular,optical laser crystals doped with rare-earth ions are widely used in the field of laser technology.Femtosecond laser inscription?FLI?is a relatively new method for waveguide fabrication,which has the advantages of better flexibility,wider material applicability and higher processing quality than other fabrication techniques.In this thesis,different optical waveguide structures are fabricated in optical crystal materials by using femtosecond laser inscription.The propagation,fluorescence,Raman,up-conversion luminescence and other characteristics of waveguides are studied.In addition,we systematically study the surface modifications of femtosecond laser irradiated on the surface of optical crystal material.According to the optical crystal materials,the main works and results of this thesis are summarized as follows:We fabricate deeply embedded optical-lattice-like structures in a Ti:Sapphire crystal by applying femtosecond laser inscription to implement two-dimensional?2D?one-to-two and three-dimensional?3D?one-to-four beam splitting.Such a family of photonic microstructures are characterized at near-infrared both experimentally and numerically,showing excellent capability of simultaneous light confinement and beam tailoring at two orthogonal polarizations.For TM transmission mode,the one-to-two beam Splitting ratio is determined to be around 0.96:1 the one-to-four beam splitting ratio is calculated to be 0.98:0.95:1:0.89.The confocal micro-Raman image of the obtained structure reveals that the optical properties of the substrate have been well-preserved in the waveguide active volumes.Our results pave a way to construct complex integrated waveguide splitters in Ti:Sapphire crystals by using FLI for photonic applications.We fabricate optical waveguide splitters with rectangular cladding geometry in a Ti:Sapphire crystal by FLI.Y-splitters are fabricated with 30?m×15?m and 50?m×25?m input ends,corresponding to two 15?m×15?m and 25?m×25?m output ends,respectively.The full branching angle?between the two out-put arms are changing from 0.5°to 2°.The performances of the splitters are characterized at 632.8nm and 1064nm,showing very good properties including symmetrical output ends,single-mode guidance,equalized splitting ratios,all-angle-polarization light transmission and intact luminescence features in the waveguide cores.The realization of these waveguide splitters with good performances demonstrates the potential of such promising devices in complex monolithic photonic circuits and active optical devices such as miniature tunable lasers.We systematically study the surface modifications of femtosecond laser irradiated Nd:YAG crystal in stationary focusing case or dynamic scanning case.Micro-sized structures are experimentally produced in a large scale of parameters in terms of pulse energy as well as?effective?pulse number.Surface ablation of Nd:YAG surface under both processing cases are investigated,involving the morphological evolution,parameter dependence,the ablation threshold fluences and the incubation factors.Meanwhile,under certain irradiation conditions,periodic surface structures with high-Spatial-frequency?<?/2?can be generated.The obtained period is as short as 157nm in this work.Investigations on the evolution of nanograting formation and fluence dependence of period are performed.The experimental results obtained under different cases and the comparison between them reveal that incubation effect plays an important role not only in the ablation of Nd:YAG surface but also in the processes of nanograting formation.Cladding waveguides with diameters of 20?m,25?m,30?m 35?m are fabricated in Yb,Na:CaF₂ crystal by applying femtosecond laser inscription.Waveguide properties are investigated in terms of guiding behaviors and confocal micro-Raman characterizations.The minimum propagation loss is about 0.5dB/cm obtained in WG1.In addition,under 946nm excitation,visible cooperative up-conversion emissions at 478nm induced by Yb³⁺ion pairs are observed while other visible bands are detected owing to the impurities of Er³⁺and Tm³⁺ions.