Research On Polarization Manipulation Device For Monolithic Integration

Monolithically integrated optical device is required to implement multiple functionalities for the development tendency of miniaturization,low cost and low power consumption of optical communication devices.Nowadays,in the coherent optical communication with long haul and high capacity,the application of dual-polarization quadrature phase shift keying modulation format demands that the integrated optical device can realize light transmitting,modulation,polarization combining,polarization splitting,phase demodulation,optical detection and other functions.The polarization manipulation device is one of the key photonic devices for the polarization multiplexing.And it is vital for improving communication capacity and suppressing the polarization mode dispersion.Widely tunable lasers,high-speed modulators,90 degree hybrids and photodetectors have been realized on indium phosphide（InP）substrate.However,the deployment of the integrated InP-based coherent transceiver is limited by the immaturity of the waveguide-type polarization manipulation device with high polarization extinction ratio（PER）and large fabrication tolerance.This dissertation mainly focuses on InP-based polarization beam splitter（PBS）and polarization rotator-splitter（PRS）for the monolithic integration of coherent transceiver.Hybrid integration is another way to achieve the integrated optical device with multiple functionalities.Therefore,realizing the PRS based on the silicon nitride（Si₃N₄）waveguide is studied in this dissertation for hybrid integration.The main research contents are as follows:（1）An InP-based PBS is proposed and realized based on Mach-Zehnder interferometer（MZI）.The electro-optic effects could produce the birefringence of the waveguide and the plasma effect could compensate the phase errors of MZI structure.The anisotropy of the In Ga As P/InP waveguide has been studied thoroughly.The phase shift and loss from the Pockels effect,Kerr effect,band-filling effect and plasma effect are analyzed theoretically and the PER is predicted with the different waveguide orientations.The mesurements of the fabricated devices indicate that only TM₀ mode can be adjusted by the reverse bias because the negative refractive index variation of Pockels effect cancels out the positive ones from Kerr effect,band-filling effect and plasma effect for the TE₀ mode.The PER is over 19 d B covering the whole C band（1530～1565 nm）when the waveguide is aligned along the[011]if one arm is biased at-5.36 V.The PER is measured to be above 18 d B on the C band when the width of the device suffers a deviation of±200nm.The measured results of the fabricated devices with different waveguide orientations demonstrate the theoretical analysis.（2）On the basis of mode coupling and mode conversion,the structure and material composition of the In Ga As P/InP waveguide is optimized,and then an InP-based PRS is proposed and fabricated on the passive waveguide.The mode converter of the device rotates the TM₀ mode into TE₁ mode and the asymmetric directional coupler（ADC）splits the TE₁ mode and TE₀ mode.The fabricated PRS has a PER of over 16 d B in the wavelength range from 1550 nm to 1620 nm.The PRS based on passive In Ga As P/InP waveguide of low birefringence is demonstrated experimentally.（3）The waveguide-type polarization manipulation devices based on the InP,silicon on insulator and Si₃N₄ platforms respectively are compared theoretically to show the advantages to realize the polarization manipulation device on the Si₃N₄ waveguide.A PRS is proposed and fabricated based on the Si₃N₄ waveguide.The ADC with high asymmetry and the 2nd mode converter are deployed to improve the fabrication tolerance and PER of the device.The measured results indicate the PRS has the PER of over 20 d B and the polarization-converting loss of around 1.5 d B across the C-band.