Research On Novel Silicon Optical Devices

Due to the large refractive index difference between the core layer and the cladding,silicon device has a very compact size and be able to realize high density integration.More complex functions,such as optical transceiving,analog-to-digital conversion and signal to noise ratio monitoring,can be realized by connecting discrete devices to form an on-chip silicon integrated photonic circuit.This kind of chip has advantages of high performance,low power consumption,high-density integration and the complementary metal oxide semiconductor(CMOS)compatibility,which is the main development direction of integrated optics in the future.This dissertation addresses researches on novel silicon optical devices,including silicon polarization splitter,silicon optical coupler and all-optical quantizer.The main research efforts are summarized as follow.1.A broadband high-performance polarization beam splitter(PBS)was demonstrated on SOI platform,which consists of a 70 nm taper-etched waveguide and a slot waveguide.For TM mode,there is phase matching between the waveguides,therefore the mode can be coupled from the taper-etched waveguide into the slot waveguide efficiently.For TE mode,there is a significant phase mismatch between the waveguides,therefore no coupling occurs between them.Based on this principle,a high performance device can be obtained over S,C and L band.Through the measurement,a polarization extinction ratio(PER)of 30 dB and 40 dB for TE and TM polarizations can be achieved at 1550 nm,with quite low insertion losses(IL)of-0.17 dB and-0.22 dB,respectively.The operation bandwidths are large as?175 nm and?120 nm for PERs of>20 dB,>25 dB,respectively,which is the largest bandwidth for these PERs reported for the SOI-based PBS to the best of our knowledge.Two-step etching process is needed for the fabrication of this device,with the etching depths of 70 nm and 220nm.Meanwhile,the structure is robust to the size variation,which means that the device can be easily fabricated in the commercial MPW service.2.A novel 1×3 MMI coupler with variable splitting ratios was proposed in this work.By cascading two MMI couplers,and modifying the structure of first-step MMI,the output distribution of the second-step MMI can be changed.Compared to other 1×3 couplers,this kind of devices have a smaller footprint,and a large operation bandwidth above 60 nm can also be obtained.Meanwhile,the fabrication process is simple since only one step etch is needed,and it is robust to the fabrication errors.Splitting ratios from 1:0.67:1 to 1:18:1 were achieved,and the measurement results also verify the stable performance with low insertion losses of below-1 dB in a large operation bandwidth over 50 nm.The lowest insertion loss is around-0.2 dB at a wavelength of 1550 nm.3.The all-optical quantizers by using 4 × 4MMI or cascaded MMI were proposed in this work.Between them,the 4×4 MMI coupler is designed and fabricated on SOI platform.Based on the measurement results,the imbalance of the four output ports at 1550 nm was around 1 dB,and an effective number of bits(ENOB)of 1.853 bit was achieved by utilizing two outputs of MMI as the quantization channels.To reduce the power consumption,a novel compact all-optical quantizer based on cascade step-size MMI structure was proposed,which utilizes odd channels as the quantization channels.A 3-bit quantizer and a 5-bit quantizer were designed and simulated based on SOI platform,the lengths of which are all below 200?m,and ILs of them are all below-0.5 dB at 1550nm.This kind of devices can be easily fabricated,and they are insensitive to the variation of structure and wavelength,which means it has potential to be very stable.