All-Optical Modulation Based On Hybrid Graphene-Microfiber Structures

As an atomic-layer 2D material,graphene has attracted extensive attention for its excellent optical and electrical properties.The combination of graphene and low loss optical microfibers has brought new opportunities for ultrafast all-optical,all-fiber modulation.Based on the optical pumping induced band filling effect,it is possible to achieve all-optical modulation with response time down to picosecond level.Here,based on systematical analysis of the opportunities and challenges of graphene for optical modulation,we optimized the performance of graphene based all-optical modulation devices from the functional structure and the working mechanism.The results may promote the practical applications of graphene based optical modulation devices and offer new ideas for the design and optimization of graphene based optoelectrical devices.In the first chapter of this thesis,a brief summary of research background of graphene in optical modulation was reviewed,including basic ideas of optical modulation technologies,optical properties and fabrication methods of graphene and microfibers,working mechanisms of graphene based optical modulation(i.e.,electric-optical,all-optical and thermo-optical).Besides,based on the free space and chip-integrated configuration,we introduced the state-of-art of graphene optical modulation.In chapter two,graphene based all-optical amplitude modulation was investigated.With an evanescent field deposition method,we locally decorated graphene flakes on the microfiber,greatly enhanced the light-matter interaction,and successfully demonstrated all-optical amplitude modulation with peak power of the switch pulses reduced by one order of magnitude.In addition,we fabricated high quality polymer nanofibers from solvated polymer doped with graphene flakes,and demonstrated a low threshold for saturable absorption with pulses of sub-pico joule.In chapter three,graphene all-optical phase mdoualtion was investigated.With polymer assistant transfer method and micro-manipulation,we covered the graphene film on the microfiber,and inserted it as one arm of an all-fiber Mach-Zehnder interferometer for all-optical phase modulation.Compared with the amplitude modulation under the same condition,the phase modulation offered a 4.6 times higher modulation depth and a 2 times higher overall transmittance simultaneously.Based on the measured data,we analyzed the trade-off between the modulation depth and the insertion loss in graphene all-optical modulation.In addition,from the output of the modulator,we found that,the optical excited carrier induced optical Kerr effect and the thermal effect from the graphene absorption can be clearly separately from the phase shift.In chapter four,we provided a detailed analysis on the future possibilities of graphene on optical modulation.We discussed the figure of merits of graphene based modulators,including modulation speed,modulation depth,insertion loss and power consumption,as well as their trade-off between each other.By comparing graphene based modulators with the commercial lithium niobate and silicon optical modulators,we discussed the challenges and opportunities of graphene saturable absorbers.Based on the experimental results,we estimated the laser induced damage threshold in the graphene-fiber hybrid structures.In chapter five,we gave a brief summary of our work,with an future outlook of this area.