Fabrication And Characterization Of Waveguide Structures In Transparent Optical Materials

Integrated optical circuits(IOC)can realize transmission and processing optical signals in a highly integrated space.IOC has been of great interest since 1960s,when it was firstly proposed,due to the small size,stable performance,low power consumption,functional diversity,et al.Various integrated photonics devices have been developed for different application,nevertheless the basic components remain unchanged,including interconnect,power splitter,waveguide reflector,directional coupler,polarizer,polarization beam splitter,phase modulator,intensity modulator,TE/TM mode converter and frequency shifter.It must be pointed that those functional components are based on the basic element:optical waveguide.Optical waveguide is a structure composed of higher-refractive-index region surrounded by a region with lower refractive index.Taking advantage of the relatively short wavelength,light signal can be confined in a space with volume in a micro or sub-micro scale.Thus,optical density is greatly enhanced inside the waveguide volume.According to the confinement and propagation of light in three-dimensional space,optical waveguides can be divided into one-dimensional(1D),two-dimensional(1D)and three-dimensional(3D)waveguide.1D waveguide is also known as planar waveguide.2D waveguide can be divided into the following types:ridge,channel and buried waveguides.Optical fiber could also be taken as 2D waveguide.3D waveguides with more complex structures,such as photonic crystal and splitters,can be used for complicated functions.2D and 3D optical waveguides can confine optical propagation and guidance in higher dimensional scale,reaching higher optical density,which means 2D and 3D optical waveguides have great application prospects in construction of compact functional integrated devices.So far,optical waveguides have been realized in several transparent optical materials,such as glass,ceramic,crystal and polymer.A few methods have been utilized to fabricate waveguides,including ion implantation/irradiation,pulsed laser deposition,ion-exchange,focused proton beam writing,epitaxial growth and femtosecond laser micromachining/writing.Femtosecond laser micromachining can realize 3D micro-processing in transparent optical materials by applying ultra-short pulse irradiation.Benefiting from the high resolution,simple and flexible process and 3D micro-processing,femtosecond laser micromachining has been emerged as one of the most efficient techniques for waveguide fabrication.In this thesis,femtosecond laser micromachining is employed to fabricate optical waveguides in transparent dielectric materials.During the processing,the energy of the laser pulse is concentrated in the focus.Different waveguide configurations can be realized by choosing appropriate parameters of the femtosecond laser,such as pulse energy,scanning velocity and separation.The content mainly includes femtosecond laser micromachining waveguides and waveguide splitters in dielectric materials;investigation of the guiding properties at a wide wavelength region and analysis the formation mechanism of waveguides;Q-switched pulse laser generation in optical waveguides based on saturable absorber;using luminescence intensity ratio to detect pump-induced thermal effect in waveguide volume.According to different dielectric materials and different types of optical waveguide devices,the main work of this thesis can be summarized as follows:We report on the fabrication of dual-line waveguides,depressed cladding waveguides,buried channel waveguides with increased refractive index femtosecond inscribed filaments,and waveguide splitters in BGO crystals by femtosecond laser micromachining.The dual-line waveguides are located at the core of two femtosecond laser filaments with a refractive index decrease of 1.4×10-3.Based on the reconstructed 2D refractive index profile of the waveguide cross section,the propagation modes of the BGO dual-line waveguides are calculated.Both the simulation and experimental results show that the dual-line waveguide supports single-mode propagation along TE and TM polarizations at the wavelength of 632.8 nm.Thermal annealing treatment is effective to reduce the propagation loss of dual-line waveguides to 0.5 dB/cm.Circle cladding waveguide with a diameter of 100?m supports multi-mode propagation along both TE and TM polarization at 632.8 nm and is insensitive to polarization.With thermal annealing at 260?,the propagation loss of the cladding waveguide is reduced to 2.1 dB/cm.It is common to fabricate waveguides with increased refractive index in glass rather than crystal.Waveguides with increased refractive index have only been reported in a few crystals,including LiNbO3,Nd:YCOB and ZnSe,with limitations in stability and polarization.By carefully choosing the fabrication parameter,waveguides with increased refractive index are realized in BGO crystal by femtosecond laser writing.The end-coupling experiment reveals that waveguides support polarization insensitive guidance at mid-infrared region.The propagation loss of the waveguide at 4?m is estimated to be?3.5 dB/cm.Then,the same femtosecond laser parameters are utilized to construct waveguide splitters with increased refractive index filaments.The 2D(1×2)and 3D(1×3,1×4)waveguide splitters support polarization insensitive guidance at 4?m,and the near-field intensity mode are single-modes.The intensity splitting ratio of the output arms are close to 1:1.The splitting losses are as low as 0.3 dB.The results show the promising capability of direct femtosecond laser written waveguide splitters in BGO crystal for MIR applications.We report on the passively Q-switched lasers in femtosecond laser written dual-line,single-and double-cladding waveguides in Nd:YVO4 crystals.Using graphene as the saturable absorber,passively Q-switched waveguide laser operations were achieved.The separation of the dual-line waveguide is 20?m.Under 820 nm CW laser pump,laser oscillations at the wavelength of 1064.4 nm have been achieved in the dual-line waveguide.The performance along TE polarization is superior to that along TM,which could contribute to the propagation loss of the dual-line waveguide:?1.2 and?1.9 dB/cm along TE and TM polarizations.At the pump power of 1.2 W,the maximum average output laser power is achieved as 129 mW,corresponding to the minimum pulse width of 25.0 ns,the highest repetition rate of 16.3 MHz and the highest single pulse energy of 8.1 nJ.Q-switched pulse laser generation is also achieved at the single-cladding waveguide with diameters of 42 or 114?m and double-cladding waveguide with inner/external diameters of 42/114 ?m.The wavelength of the generated laser is 1065.2 nm.The cladding waveguides possess more symmetrical shape,lower loss and better laser performance.Especially,the double-cladding waveguide is superior to the single-cladding waveguide,as the outer cladding provides large pump area and single-mode propagation is achieved by inner cladding confinement.The maximum waveguide laser output power is?361 mW with a lasing threshold of 103 mW along TE polarization,meanwhile,the repetition rate of the pulsed laser could be tuned and raised up to 14.5 MHz corresponding to single-pulse energy of 24.9 nJ and pulse width of 31 ns.We have demonstrated how luminescence thermometry,in combination with high resolution confocal fluorescence imaging,constitutes a contact-free technique for intra-waveguide thermal imaging.Femtosecond laser writing is utilized to fabricate channel waveguides in Nd:YAG crystal and Er:Yb:phosphate glass.For 800 nm injected laser powers close to 100 mW,intra-waveguide laser-induced heating of a few degrees have been experimentally found in Nd:YAG waveguide.Both the theory and experimental results show that:i)the refractive index of femtosecond laser inscribed filaments decreases,thus the light is confined in the waveguide core;ii)femtosecond laser induce filaments are also characterized by a reduced thermal conductivity,acting as the thermal barriers,so thermal loading is mainly confined at the waveguide volumes.The luminescence measurement of the Er:Yb:phosphate glass waveguide is realized by the two-photon excited luminescence of Erbium ions.The thermal effect of the waveguide is much obvious as the high thermal sensitivity and low glass thermal conductivity.In combination with high resolution multiphoton fluorescence imaging,high resolution 3D thermal imaging of Er:Yb:phosphate glass waveguide is obtained.Under 120 mW,980 nm CW laser pump,the temperature increments at waveguide volume is as large as 200?,which means that the laser induced thermal loading cannot be neglected at all.To reduce the thermal effect,time modulation is applied to change the pump laser to a square pulse laser,with a fixed pulse width of 2 ms and fixed peak power of 60 mW.Experimental and numerical simulations have concluded that intra-waveguide temperature increment can be reduced by increasing the pulse separation,as the heat accumulation between neighbor pulses can be minimized.We fabricated cladding waveguides in LiNbO3 crystal by using femtosecond laser micromachining,and studied the effect of propagation direction and diameter on the guiding property.The cladding structures with diameter of 50?m and 110 ?m support guidance along both extraordinary index(ne)and ordinary index(no)polarizations at wavelengths of 0.633 ?m,1.064 ?m and 4 ?m.At the wavelength of 4 ?m,single modes are achieved for the cladding waveguides with diameter of 50 ?m.After thermal annealing,the insertion loss of the cladding waveguide with a diameter of 110?m(along z axis)was reduced to 0.5 dB.