Design And Fabrication Of Novel Bragg Gratings With Multiple Phase Shifts

Tunable semiconductor lasers and dispersion compensators are key devices for wavelength division multiplexing(WDM)systems.As an important element of these two devices,Bragg gratings have been widely concerned by many researchers.Among these gratings,novel Bragg gratings with multiple phase shifts are attractive for use in tunable semiconductor lasers and dispersion compensators,due to their good characteristics in reflection spectrum.In terms of all fabrication techniques of Bragg gratings,nanoimprint lithography(NIL)with the advantages of high resolution,low cost and high yield is very suited for fabricating novel Bragg gratings with multiple phase shifts.The thesis focuses on designing and fabricating novel Bragg gratings with multiple phase shifts,and the detailed works are summarized as follows:(1)Based on coupled-mode theory and transfer matrix method,a model for analyzing reflection spectrum and dispersion of Bragg gratings is developed.By using the model,reflection spectrums of sampled grating(SG),sampled grating with multiple phase shifts(MPS-SG)and digital concatenated grating with multiple phase shifts(MPS-DCG)are analyzed.For the application of widely tunable semiconductor lasers,MPS-SGs and MPS-DCGs on waveguide are designed.For the need of dispersion compensation for multiple channels,dispersion compensators based on digital concatenated chirped-sampled fiber grating with multiple phase shifts(MPS-DCCSFG)are proposed and designed.(2)Process of NIL are simulated by using solid module in Comsol.By the simulation,relationships between filling degree of resist and parameters in the NIL process are studied.The results provide a guidance for parameter optimization in the NIL process.(3)Fabrications of MPS-DCGs on waveguide using NIL are studied.By introducing soft stamp technology and multi-step changes of pressure and temperature,MPS-DCG patterns are demonstrated using NIL.Measured results show that the imprinted patterns have good uniformity,and parameters are consistent with the designed values.After optimizing the etching process,MPS-DCGs are well fabricated on Si substrate.And the period,depth and phase shift of the grating are satisfied,whlie the sidewall is vertical.