Integrated LiNb₃Waveguide E-field Sensor Based On Asymmetric LiNb₃M-Z Interferometer

Integrated electro-optical sensor is becoming more and more attractive because ofits high sensitivity, large bandwidth and no interference to the original electric field. Aswe all know, the operating point of integrated electro-optical sensor is the key factor ofthe device’s performance and has a significant impact on the sensitivity, linear dynamicrange and signal distortion in the sensing system. Moreover, it will become unstablewith the fabrication tolerance and the environmental factors, such as temperature,humidity, stress. To solve this problem, an optical waveguide electric field sensor withcontrollable operating point is proposed and fabricated in this dissertation, which has adirect asymmetric Mach-Zehnder interferometer (MZI) optical waveguide structurefabricated with LiNbO3(Lithium Niobate) materials and uses a tunable laser as a lightsource.First, the operation principle of the aforementioned optical waveguide electric fieldsensor is studied and its sensing performance is analyzed. Theoretical results show thatthe operating point can be located in an excellent linear region by tuning the outputwavelength of the tunable laser used in the sensing system. The simulation results showthat the sensitivity of~83dBμV/m can be obtained and the linear dynamic range aslarge as62dB can be achieved. And the fabrication tolerance of the center-to-centerdistance for the3dB coupler used in the asymmetric MZI is~0.5μm, while the powersplitting ratio of the Y branch has more tolerance.Then, a retroreflective asymmetric MZI optical waveguide electric field sensor isdiscussed briefly, which is realized by cutting the Lithium Niobate substrate at the twoarms of MZI structure with a certain angle, then polishing the cutting face, and at last,depositing a layer of reflective film at the cutting face. It makes the light propagated inthe two arms of MZI structure reflect at the ending face and then interfere at the inputport of waveguide. Hence, this structure can make the length difference of two armsobtained tactfully by changing the angle between waveguide and cutting face.Compared with the direct asymmetric MZI structure, it has a smaller size and canachieve the length difference of two arms easier. But its fabrication cost is much higher. Finally, considering the fabrication complexity, the sensor with a direct asymmetricMZI structure is fabricated based on the parameters of the device designed above. Andthe optical transmission characteristics and sensing performance of the device arecharacterized. The results show that the output power of the device is similar tosinusoidal periodic function and it can cover half of the period basically in C band(1530nm-1570nm) which is consistent with simulation results and the sensingcharacteristics at different working wavelength verified the feasibility to control theoperating point by changing the operating wavelength in the sensing system.