Research On Several Key Problems Of All-Optical Analog-To-Digital Conversion

Analog-to-digital converter (ADC) is the key front-end interface device of ultra-wideband digital systems. The advent of huge amounts of information age puts forward austere challenge for existing electronic ADCs. All-optical ADCs digitize the analog signal by photonic technologies and can fundamentally resolve the problems of inherent electronic bottleneck of the microelectronic technologies. Thereby, they can achieve much higher digitization performances than the traditional electronic ADCs. Through the reported all-optical ADCs achievements, some key problems like efficient multiplexing of sampling rate, combination of large analog bandwidth and high quantization resolution, integration, miniaturization and low power consumption still exist and need to be solved. Aiming at these key problems, the implementations of integratable and high performance all-optical ADC using the third-order nonlinear optical effects and electro-optical modulation techniques are mainly studied in this thesis. The major contents and the innovating results are listed below:(1) The principle of optical sampling based on four-wave mixing (FWM) effects is studied. And, the novel chirped multicast parametric synchronous optical sampling scheme is proposed. In the case of only three multicast copies, the sampling rate of 120 GSa/s is achieved using the mode-locked laser diode (MLLD) with only 10 GHz pulse repetition rate. The multiplexing ratio is up to 12 times, which is far beyond the results of the traditional multicast parametric synchronous sampling schemes. This innovation can resolve the interacting of the timing jitter and repetition rate of MLLDs, which means hundreds of GHz sampling rate can be expected using low timing jitter and repetition rate MLLDs.(2) The all-optical frequency quantization approaches using the third-order nonlinear optical effects in special fibers are studied. And, the frequency quantization scheme employing the soliton self-frequency shift (SSFS) effect in a photonic crystal fiber (PCF) is proposed. The designed PCF shows high nonlinear coefficient of 62.8W-1/km at 1550 nm, with which the large wavelength shift up to 318 nm is obtained in the simulation. Further, a 100 m dispersion-increasing fiber (DIF) is used to compress the pulse spectrum to about 4 nm after the SSFS process. And, the final quantization resolution could be 6-bit. Comparing to the traditional SSFS quantization schemes with highly nonlinear fiber (HNLF), the proposed scheme exhibits higher performances and lower structure complexity.(3) Aiming at the inherent time-delay issue of the SSFS quantization schemes, a lumped time-delay compensation method is proposed using flattened negative dispersion fiber (NDF). The proposed method can compensate the time-delays of all quantization channels together, which reduce the structure complexity dramatically in high resolution cases. However, the GVD influence of the NDF broadens the pulse widths simultaneously during the time-delay compensation, which limits the maximum supportable sampling rate to 30 GSa/s. To solve this problem, the lumped time-delay compensation method using a linearly chirped fiber Bragg grating (LCFBG) is proposed. Through optimizing the chirp variable, modulation depth, and apodizing functions, the compensation error lower than 3.3 ps is obtained, which improves the supportable sampling rate up to 151.52 GSa/s.(4) All-optical quantization methods using the efficient nonlinear effects in the novel silicon-on-insulator (SOI) waveguide are studied and a CMOS-compatible 2-bit optical spectral quantization scheme using a silicon-nanocrystal (Si-nc)/SiO2 based horizontal slot SOI waveguide is proposed. The ultra-low effective area of 0.027 μm2 at 1550 nm is obtained by optimizing the waveguide structure, which provides the high real part of nonlinear coefficient up to 8708 W-1/m. Utilizing only 8 mm long waveguide, the effective number of bit (ENOB) of 1.98-bit is obtained in the case of only 0.4 W peak power threshold. The ultra-low threshold makes the proposed method non-affected by the coupling loss issue and can be optically interconnected with other SOI functional devices on the same chip. Further, two all-optical frequency quantization schemes based on the As2S3/silicon horizontal slot waveguides are proposed. The two shemes are using SSFS and supercontinuum effects, respectively, and achieve 2.72-bit and 3.98-bit ENOB in the cases of 5 cm long with 23 W peak power threshold and 1.5 cm long with 0.9 W peak power threshold. The studies in this chaper provide the research basis for the implementation of integratable and low power consumption all-optical ADCs.(5) The compact and low loss of 0.065 dB/μm optical Bragg reflector and micro-cavity are proposed using only 7.88 μm long dielectric loaded surface plasmon polariton waveguide (DLSPPW). In the case of 15 dielectric periods, the bandwidth of the Bragg reflector reaches to 375 nm with the 1% transmissivity of stop-frequency gap. Meanwhile, the quality factor of the optical micro-cavity reaches 774. The performance parameters of the proposed Bragg reflector and micro-cavity can be tuned by optimizing the waveguide structure so that they are expected to be used in the all-optical ADC schemes as the broadband time-delay compensator and filters.(6) An all-optical two-stage cascade quantization scheme is proposed to enhance the quantization resolution. The first quantizer of the scheme is a unbalanced Mach-Zhender modulator (UMZM) while the second quantizer is a GaAs/AlGaAs asymmetric directional coupler with 2-bit resolution. The directional coupler exhibits three stage power splitting with the splitting ratios of 1:1,1:1, and 2:1, respectively. In the case of 8 channels, the resolution of 5.59-bit is realized, which shows the 1.59-bit resolution enhancement. The simulation results show the SNR and ENOB are 33.58 dB and 5.28-bit, respectively. With further investigation of practical noises influences, the analog bandwith of the scheme is confirmed to be more than 50 GHz. Noticing that the key components utilized can be all realized by silicon photonics technologies, the proposed scheme has great integration potential. Moreover, an improvement of the proposed cascade quantization shceme is proposed. By cascading two UMZMs, the period of the power transfer function is doubled within the same voltage range. The high resolution up to 8.59-bit can be obtained with 16 channels and 4 periods, which presents 2-bit resolution enhancement than the previous cascade quantization scheme. The preliminary simulation shows the ENOB can reach 8.18-bit, which lays a foundation for the implementation of high resolution all-optical ADCs.The research achievements above can rich the theory of all-optical ADC technologies and make base for the applications of nonlinear silicon photonics devices in all-optical ADCs. The results provides theoretical research basis for the implementation of integratable, miniaturization, low power consumption, and high performance all-optical ADCs, which are expected to be extensively implemented in the future ultra-broadband optical communication networks and high-speed information processing fields.