Research Of Lithium Niobate Optical Waveguide Devices For The Applications Of Mode-Division-Multiplexing System

The internet traffic has sustained exponential growth in the past few decades,and this trend is expected to continue for the foreseeable future.However,it is well known that the capacity of a communication channel cannot exceed the Shannon limit.Due to the nonlinear effects of fused fiber,the traffic capacity of single-mode fiber(SMF)is limited to about 100 Tb/s.Recent experimental SMF-based transmission set-ups almost approach this limitation.The SMF-based transmission is faced with‘capacity crunch'.To sustainedly meet the traffic demand,the space-division multiplexing(SDM)attrach much interest in the context.The mode-division multiplexing(MDM),which exploits the spatial modes of fiber or optical waveguide as independent signal channels,is one of the essential components of the SDM system.The MDM technology is capable of advantages of high information density and low power consumption.Long-haul MDM transmissions based on few-mode fiber(FMF)or multi-core few-mode fiber(MC-FMF)have been already experimentally demonstrated.However,most reported MDM demonstrations to date have consisted of point-to-point transmission.Today's flexible photonic mesh networks are reconfigurable,which enable carriers to establish light paths remotely and switch those light paths on demand.For the construction of reconfigurable MDM networks,active mode controlling devices such as add-drop mode multiplexers or mode switches,reconfigurable mode(de)multiplexers,mode routers are required.Most reported active mode controlling devices to date are based on micro-electro-mechanical-system(MEMS)and thermal-optic(TO)effects,which provide switching responses on the order of millisecond or microsecond.Compared with MEMS-based devices or TO devices,the optical devices based on electro-optic(EO)effect have the remarkable advantages of fast switching response(on the order of nanosecond or picosecond).There are few reported EO mode controlling devices and hence it's of great significance to develop new various kinds of EO mode controlling devices to meet the demand for the construction of high-speed reconfigurable MDM networks.Lithium niobate(LN)is an intrinsic EO material with stable properties,low optical absorption loss and large EO coefficients.The LN-based EO devices have the advantage of low power consumption.The LN optical waveguides fabricated by the Ti-diffusion or annealed proton exchange(APE)process have achieved featured commercial applications such as high-speed EO modulators and EO switches.In recent years,the LN thin film on insulator(LNOI)platform attracts much attention.For the LNOI platform,the submicrometre-thick LN film is bonded on top of a low-index substrate such as silicon dioxide,and the optical waveguides can be fabricated by dry etching.The LNOI waveguide has high index contrast and tightly confined optical modes.The LNOI photonic devices provide high unite integration density and promising electro-optic efficiencies.Based on the conventional APE-LN waveguide platform and the LNOI waveguide platform,a series of EO mode controlling devices are proposed and studied in this dissertation to meet the requirement of developing high-speed reconfigurable MDM networks.The innovations of this dissertation are summarized as follows:1.A reconfigurable add-drop mode multiplexer or a mode switch formed with the structure of tunable asymmetric directional coupler based on APE-LN waveguides is proposed.Three stand-alone EO mode switches have been experimentally demonstrated to achieve the reconfigurable add-drop multiplexing of the LP_11a mode,the LP_11b mode and the LP₀₂ mode in a FMF,respectively.For the wavelength 1550 nm,the insertion losses of the three devices are less than 5.0 d B,4.0 d B and 5.5 d B,respectively,the mode-dependent losses are less than 0.9 d B?1.0 d B and 1.5 d B,respectively,the modal crosstalks are less than-12.5 d B,-19 d B and-25.0 d B,respectively,and for the C-band,the mode coupling efficiencies are more than 67%,40%and 56%,respectively.In the meanwhile,an integrated three-mode switch formed with two cascaded dissimilar tunable asymmetric directional couplers has been also demonstrated,which can simultaneously achieve the reconfigurable add-drop multiplexing of the LP_11a mode and the LP_11b mode in a FMF.For wavelength 1550 nm,the insertion loss is less than 9.0 d B,the mode-dependent loss is less than 5.0 d B,the modal crosstalk is less than-16.0 d B,and for the C-band,the mode coupling efficiency is more than 69%.The switching response of the integrated three-mode switch is about 200 ns to 300 ns,which are several orders(2 to 5)of magnitude shorter than than those MEMS-based devices or TO devices.Besides,to compensate for the fabrication errors of APE process,a passive post-tuning technique is proposed to improve the performances of a mode switch.Moreover,a broadband mode switch is designed and stimulated.2.A mode pattern converter based on a buried LN waveguide fabricated by a bonding process for the mode coupling between a FMF and an APE-LN few-mode waveguide is proposed.A typically sample can convert three lowest-order APE-LN waveguide modes to three fiber modes,which are the LP₀₁ mode,the LP_11a mode,and the LP_11b mode,respectively.Besides,a vertical directional coupler formed with a buried bonded few-mode waveguide and another APE single-mode waveguide placed on top was proposed to achieve the mode-selective coupling.Two fabricated vertical directional couplers have achieved the mode coupling for the LP_21b mode and the LP₀₂mode,respectively.For the C-band,the mode coupling efficiency for the LP_21b mode is less than 56%and that for the LP₀₂ mode is less than 18%.3.The EO mode controlling devices based on the LNOI platform are proposed.Two kinds of devices based on etched LNOI waveguides have been experimentally demonstrated,which are a reconfigurable two-mode(de)multiplexer formed with two cascaded Mach-Zehnder interferometers(MZI),and a mode switch formed with the structure of tunable symmetric two-mode directional coupler.The reconfigurable two-mode(de)multiplexer achieved the reconfigurable mode multiplexing and demultiplexing of the TE₁₁ mode and TE₂₁ mode with an efficiency more than 98%for C-band.For the wavelength 1550 nm,the insertion loss is less than 14.5 d B,the mode-dependent loss is less than 0.2 d B and the modal crosstalk is less than-16.9 d B.The switching response is about 20 ns to 30 ns,which are several orders(3 to 6)of magnitude shorter than than those MEMS-based devices or TO devices.The mode switch achieved the reconfigurable add-drop mode multiplexing of TE₂₁ mode with an efficiency more than 65%for the C-band.For the wavelength 1550 nm,the insertion loss is less than 16.6 d B,the mode-dependent loss is less than 4.2 d B and the modal crosstalk is less than-30.0 d B.The experimental results show that the fabricated EO mode controlling devices based on the LNOI waveguide have the remarkable advantages of compact device size,fast switching response,large EO efficiency and broad wavelength bandwidth.