| Analog-to-digital converter(ADC)built a bridge between the real word and digital world,and plays an important role in the modern information society.Over the years,with the development and application of high-speed signal processing technology in communication,medicine,military and other fields,high sampling rate,high quantization resolution and wide bandwidth are required for ADCs.However,limited by the two bottlenecks of timing jitter and compotator blur,the traditional adcs can’t handle with the contradiction between sampling rate and quantization resolution,thus can’t achieve higher performance.The optical analog-todigital converter(OADC)is proposed to overcome these bottlenecks.OADC follows the trend of future development,it refers to the utilizing of photonic technologies for sampling and quantization,which can not only overcome the obstacles of electronic bottlenecks,but also facilitate the realization of optical integration on chip.For the quantization process,one of the key points of analog-to-digital conversion,the optical phase quantization technology has attracted the attention of researchers due to its advantages of stability and integration.In the background of silicon photonics,phase quantization can be accomplished on chip with the use of silicon passive devices.Phase quantization will not limited by the sampling bandwidth and sampling rate,and the fabrication process is mature and stable,which make it the most suitable quantization scheme for integration,and has important theoretical significance and practical value.This paper mainly explores the on-chip phase quantization technology,the main research efforts and innovations are as follow:1.The cascaded step-size multimode interferometer(CS-MMI)based optical phase quantizer is studied,the fabricated chip is tested comprehensively and the experimental results are analyzed.The results show that the ENOB is 3.3 1bit at the center wavelength,and the operation bandwidth is 12mn under the condition of ENOB>3bit.According to the working principle of CS-MMI,the MMI2nd and MMI1st are tested separately.The simulation results are analyzed,which verify that the narrow operation bandwidth of ENOB is caused by the wavelength depending feature of MMI1st.In order to solve the problem of bandwidth limitation of ENOB,the wavelength feature of propagation constant of MMI1st is analyzed theoretically.According to theoretical derivation,it is found that the wavelength depending feature of the propagation constant is related to the wavelength feature of the material and inversely proportional to the square of the waveguide width.Finally,the method of using SiN3 to broaden the ENOB operation bandwidth is proposed.The simulation results show that the operation bandwidth is broaden to 43nm under the condition of ENOB>3bit.2.In order to solve the problems such as poor imaging quality and narrow ENOB operation bandwidth existing in the traditional CS-MMI based phase quantizer,three CS-MMI phase quantizers with new structure which can effectively improve the imaging quality are proposed.By establishing the exact expression of the propagation constant,a method is proposed which can effectively reduce the error between the propagation constant value in the multimode region and the ideal theoretical value by reasonably designing the side refractive index of the MMI,so as to improve the image quality and the performance(insertion loss,imbalance degree and operation bandwidth)of CS-MMI.Then,the side structure of CS-MMI phase quantizer is optimized by using this method.Through simulation verification,CS-MMI phase quantizer based on side subwavelength gratings(SWG)structure,CS-MMI phase quantizer based on side shallow etched SWG structure and CS-MMI phase quantizer based on rib waveguide structure are realized respectively.The simulation results show that compared with the traditional CS-MMI phase quantizer based on rectangular waveguide structure,the three new CS-MMI phase quantizer show effective performance improvement((insertion loss is reduced by at least 0.75dB,unbalance degree is reduced by at least 0.94dB,operation bandwidth is increased by at least 58nm under the condition of ENOB>3bit).Among them,the CS-MMI phase quantizer based on rib waveguide has simple structure and is compatible with MPW fabrication technology,which is the most feasible improvement scheme.Compared with the performance of traditional CS-MMI phase quantizer(insertion loss is 1.26dB,unbalance degree is 1.60dB,ENOB operation bandwidth is 12nm),the simulation results show a significant performance enhancement,which indicate the insertion loss of 0.24dB,unbalance degree of 0.21 dB and ENOB operation bandwidth of 70nm.Finally,a CS-MMI phase quantizer based on thin-film lithium niobate rib waveguide is proposed to realize the monolithic integration of phase modulation part and phase quantization part in OADC chip.Using this quantizer,the sampling and quantization process can be realized on a silicon Lithium Niobate thin film chip,which fits the all-optical OADC scheme with high integration degree.Simulation results show that the quantizer has excellent performance with insertion loss of 0.36dB,unbalance degree of 0.09dB and operation bandwidth of 64nm under the condition of ENOB>3 bit.3.Based on the device level,three quantization resolution enhanced schemes are proposed.The first one is directly increasing the number of output ports of CS-MMI phase quantizer.A 9-port CS-MMI phase quantizer is designed and fabricated,the experimental results show that the ENOB is 4.11bit(the theoretical quantization resolution is 4.17bit).The second one is a parallel system of double 5-port CS-MMI phase quantizer.The system is designed and manufactured,the chip packaging measurement is carried out and the experimental results show that the ENOB is 4.24bit(The theoretical quantization resolution is 4.32bit).The third on is a 1 bit enhanced system using a new phase modulation structure.In the structure,a reference waveguide is added to the traditional push-pull phase modulation structure.This method can increase the 1 bit quantization resolution by using only one channel instead of doubling the number of output ports,which can reduce the complexity of optical path and backend circuit.The theoretical quantization bit is 4.32bit.Based on the algorithm level,a quantization resolution enhanced scheme by combining the unlimited sampling recovery algorithm and phase quantizer is proposed.Thanks to the periodicity of the phase of light,the algorithm is used to process the quantization results of multiple phase periods to obtain a higher dynamic range and increase the quantization resolution.Experimental results show that this method can effectively increase the ENOB of the phase quantizer,and the ENOB can be increased by 1bit every time the phase period is doubled. |
