| Lithium niobate(LN)crystal is a multifunction material with many excellent physical properties,such as electro-optic,acousto-optic,photo-elastic,photorefractive,nonlinear optic,piezoelectric,dielectric,ferroelectric,and pyroelectric properties.LN also has a good mechanical stability,a broad intrinsic optical bandwidth and a broad optical transmission range.LN is widely used in the nonlinear optics and integrated optics.Recently,lithium niobate thin film(Lithium niobate on insulator,LNOI)fabricated by ion implantation and wafer bonding technology shows that it has similar physical properties as those of the bulk LN materials.Due to the high refractive index contrast between the LN film and the SiO2 layer,devices fabricated on LNOI have shown better confinement of light,smaller dimensions,and higher integration density than those of bulk LN.A variety of excellent devices have been fabricated on LNOI,such as low-loss waveguides,electro-optic modulators,second harmonic generations,and multiple optical frequency conversion devices with periodically poled lithium niobate.The coupling between devices on LNOI and fiber is an important topic in integrated optics.Some coupling methods can be used to coupling light from fibers to waveguide devices in LNOI,such as end-face coupling and surface coupling.The cross-section diameter of a waveguide on LNOI is around 0.5 ?m,while the diameter of a standard single-mode fiber core is around 8-10 ?m.The mode mismatch between the fiber and the waveguide makes the efficient coupling difficult.By using a tapered waveguide or a lensed fiber,the mode mismatch and the coupling efficiency can be improved.The end-face coupling needs a facet polishing of the sample,and sometime an extra focused len or lensed fiber,which increases the complexity of the system.A grating coupler made in waveguide can improve the mode matching between the diffracted optical field and the fiber mode.Grating couplers have the advantages that can be etched or adhered in anywhere of a chip,need no post-polishing of waveguide or after-treatment of the fiber,which can fulfill the coupling and testing requirements of integrated optics.Many methods can be used in fabricating gratings,such as reactive ion etching(RIE),Ar+ ion etching,and inductively coupled plasma etching(ICP).Grating structures are patterned generally by UV photolithography or electron beam lithography,and so the accuracy of grating is highly dependent on the resolution of lithography process.In focused ion beam(FIB)system,the electrostatic lens focus the ion beam to realize etching and microscope functions.FIB is a direct micromachining technology,which needs no lithography.By gathering the secondary electrons with low energy,FIB can take micrographs during the etching process.The etching resolution and accuracy is related to the intensity distribution of beam current.In this dissertation,grating couplers are etched by FIB.Currently,a few researches on grating couplers on LNOI have been reported.From year 2015 to 2017,grating couplers on LNOI with a measured coupling efficiency of-12 dB,-9.5 dB,and-10 dB have been reported by M.Mahmund in Carnegie Mellon University,M.S.Nisar in Huazhong University,and M.A.Baghban in Royal Institute of Technology,respectively.More efforts are needed to obtain effective,broadband grating couplers with good fabrication tolerance.In this dissertation,waveguide grating couplers have been designed,fabricated and characterized.The main contents and results are as follows:1.Uniform grating coupler on LNOIThe period of grating couple on LNOI is calculated using the Bragg condition.Simulations are performed by two dimensional finite difference time domain(2D-FDTD)method.The grating parameters,such as period,etch depth,LN thickness,SiO2 thickness,fiber angle and fiber positions are studied systematically.The results show that the coupling efficiency is depended on all these parameters,and an optimized coupling efficiency of-3.5 dB is obtained at wavelength 1550 nm when the LN thickness,the period,the filling factor,and the etch depth is 400 nm,980 nm,0.5,and 225 nm,respectively.A grating coupling testing system is setup to test the coupling efficiency.The measured coupling efficiency is-9.2 dB at wavelength 1550 nm,and the 3 dB bandwidth is 96 run.2.Grating coupler in LNOI with a metal bottom reflectorWaveguide grating coupler on LNOI with a metal bottom reflector is designed to use the reflectivity of metal material.Grating couplers in z-cut and x-cut LNOI are optimized,and results show that the optimized grating coupler in z-cut LNOI platform has a smaller period,a deeper etch depth than those in x-cut LNOI.The reason is attributed to the refractive index difference of the transmitted TE mode.The simulated coupling efficiency of-1.1 dB is obtained at wavelength of 1550 nm in Z-cut LNOI when the LN thickness,Si02 thickness,the metal thickness,the period,the filling factor,and the etch depth is 400 nm,1.275 ?m,100 nm,928 nm,0.5,and 160 nm,respectively.Experimentally,LNOI with a metal bottom reflector is fabricated,and waveguide gratings with different waveguide lengths are etched by FIB.The coupling efficiency is measured in the wavelength range from 1500 nm to 1600 nm.Results show that grating coupler with a 3 dB bandwidth of 72 nm and a coupling efficiency of-6.9 dB at 1550 nm.The coupling efficiency is higher than those of the reported grating couplers in LNOI.3.Chirped(non-uniform)grating couplers on LNOIThe coupling efficiency could be improved by chirping the period and the filling factor to make a better matching between the diffracted optical field of the grating and the Gaussian mode profile of the fiber.A non-uniform grating coupler is designed based on this mechanism.The non-uniform grating coupler consisted of 18 grating periods,and the first nine periods and filling factors are adjusted to make a better mode match to the Gaussian mode profile of the fiber.The fabrication tolerances of uniform and non-uniform grating couplers are studied and compared.The simulated results show that the declination of coupling efficiency caused by fabrication errors in non-uniform grating coupler is lower than that in uniform grating coupler,but the wavelength shift is bigger.The simulated maximum coupling efficiency of the non-uniform grating coupler is-1.4 dB and-0.02 dB without and with a metal bottom reflector,respectively.The non-uniform grating couplers are fabricated by FIB on the z-cut LNOIs with and without the metal bottom reflector.The LN thickness without a metal reflector is 494 nm,and LN thickness with a metal reflector is 480 nm.The measured coupling efficiency,the 3 dB bandwidth for the non-uniform couplers without or with metal reflectors is-6.9 dB or-5.5 dB,90 nm or 84 nm,respectively.The fabrication process on the non-uniform grating couplers simple,and needs no extra deposition or lithograph process.The non-uniform grating coupler is a promising coupling approach.4.Silicon grating coupler on LNOIThe silicon(Si)material with excellent electrical properties has unparalleled applications in integrated circuit.If LN can integrate with Si,which can combine the electrical properties of Si and optical properties of LN,more compact and effective optic-electric devices are expected.Two kind of Si grating couplers are designed on LNOI.One is with Si strips on LNOI,called the Si-strip-loaded grating coupler.The simulations are performed with different LN and Si thickness.When the LN thickness is fixed,the coupling efficiency is varied with the Si thickness,and a thicker Si stripe grating has a smaller period and a deeper etch depth.When the Si thickness is fixed,the coupling efficiency is decrease with the increase of LN thickness.The optimized coupling efficiency is about-4.3 dB when the LN thickness is 300 nm,and the Si thickness is 50 nm.In experiment,a lift-off process is used to form the Si-strip-loaded LNOI waveguide,and then FIB is applied to etch the gratings.The coupling efficiencies in the wavelength range from 1420 nm to 1580 nm are measured.The measured coupling efficiency is-12.8 dB at wavelength 1500 nm.The second kind of Si grating coupler is formed by loading a Si sheet with grating on LNOI.The optimized coupling efficiency is-1.6 dB when the Si thickness,the etch depth,the period,the filling factor,the period variation,the filling factor variation,and the dimension of Si grating is 320 nm,320 nm,840 nm,0.24,0.01 ?m,0.04,and 12×18?m2,respectively.The Si grating couplers provide an efficient coupling solution on integrated electro-optical chips with high efficiency and high density. |
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