The Research Of SOI-Based Variable Optical Attenuator Used In Microwave Photonic Phase Shitfer

Variable Optical Attenuator￡¨VOA￡is used to tune optical power dynamically, and playsan essential role in fiber optical communication especially in Wavelength DivisionMultiplexing (WDM) system. SOI based VOA can be easily integrated with otherfunctional photoelectric devices and be widely researched. We designed and fabricated theSOI based VOA, which is used in Microwave Photonic Phase Shifter (MWPPS), with largeattenuation, insensitive to wavelength, small size, simple structure, and can be easilyembedded into integrated MWPPS’optical waveguide delay lines.In the first place, this paper expounded the principle of MWPPS based on the vector-sum,and constructed the phase shifter test system, which composed of discrete devices. In thistest system,10GHz microwave signals’350°phase-shift tuning range can be obtained. TheMWPPS using the structure of the two branches can achieve a large range of phaseadjustment by applying the reversed-phase characteristic of Mach-Zehnder electro-opticintensity modulator. VOA’attenuation values directly determine the phase tuning range ofthe phase shifter.The VOA consists of SOI based single mode waveguide, multimode waveguide, and thinfilm electrode covered on multimode waveguide. During our designing, considering thetolerance of fabrication,5.2μm top silicon was shallow etched of900nm to form singlemode waveguide.2D and3D BPM simulation was conducted to make sure the properwaveguide dimension and verify the functionality of VOA. Through simulation andanalysis, multimode waveguide of20μm width and600μm length was decided.Simultaneously, the thermal-optic tuning zone which consists of aparallelogram modulation area with the tilt angle of3degrees and a S-Bend Arc Tapermodulation area was eventually identified by simulation and analysis. The temperaturedistribution of the heating area is simulated by finite element analysis software in thecondition of Ti and Pt electrode. The result of35degrees temperature razing needs2.1and1.95watt power consumption, corresponding15.4V and7.2V external voltage respectively.Lithography, etching, sputtering, and stripping is adopted to make devices. The process ofis focused, and the performance of the VOA is tested afterwards. The maximum attenuation is28dB, which can satisfy the demand of MWPPS. The fitted cure of optical powerattenuation matches well with BPM simulation. The power consumption of1.33W at28dBattenuation and thermal simulation quantities are of the same order of magnitude. Afteroptimization of performance, the VOA can be applied in many areas of opticalcommunication system.