Research On Key Technologies Of Optical Analog-to-digital Convertor

Broadband,high-speed and high-resolution analog-to-digital convertors(ADCs)play a significant role in numerous applications such as ultra-wide radar systems,electronic countermeasures,wireless communications,and broadband signal real-time detection.Conventional electronic ADCs possess a relatively large improvement in the sampling rates,whereas their performances of analog bandwidth and time jitter have been close to the limitation mainly restrained by the integrated material.Optical ADCs utilizing the ultra-high speed,ultra-wide bandwidth and ultra-low time jitter optical technology,are recognized as a promising candidate towards high-speed,broadband and highresolution analog-to-digital conversion.This thesis focuses on optical ADC to study the existing key technical problems theoretically and experimentally.Additionally,the optical ADC is applied to microwave photonic frequency measurement.The main contents of the thesis are summarized as follows:(1)An optical ADC is proposed based on cavity-less optical pulse source and optical undersampling,aiming to improve sampling rate and broaden analog bandwidth simultaneously.The cavity-less optical pulse source consisting of a Mach-Zehnder modulator(MZM),two phase modulators(PMs)and a section of single-mode fiber(SMF),generates a nearly chirp-free ultra-short optical pulse train with high and tunable repetition rate and low time jitter to undersample the input broadband analog signal,which is digitized by low-speed electronic ADC.Both numerical simulation and experiment are implemented to evaluate the feasibility of the proposed scheme,where the40 GHz microwave signal is undersampled by a nearly chirp-free ultra-short optical pulse train with the repetition rate at 3 GHz.Besides,a broadband high-resolution microwave frequency measurement based on optical ADCs via using three cavity-less optical pulse sources with different wavelengths and coprime repetition rates is proposed.The input microwave signal is down-converted into three different intermediate frequency(IF)signals located in the first Nyquist range.Based on the proposed frequency identification algorithm,the input microwave frequency can be retrieved from the IF signal frequencies obtained by fast Fourier transform(FFT)calculation of the digitization data from three electronic ADCs.In the proof-of-concept experiment,a deadband-free microwave frequency measurement up to 40 GHz is realized via utilizing three cavity-less optical ultra-short pulse sources with repetition rates of 2.99 GHz,3.07 GHz and 3.10 GHz,where the measurement error and the frequency resolution are ±5 k Hz and 10 k Hz,respectively.(2)An optical ADC scheme based on cavity-less optical pulse source and parallel multichannel time-demultiplexing electronic quantization technique is studied,which can simultaneously realize a high sampling rate and circumvent the speed mismatch between the sampling optical pulses and the electronic ADCs.The high-repetition and low-timejitter optical pulses generated from the cavity-less optical pulse source undersample the broadband analog signal.Then,the optical pulses are taken apart into several parallel lowspeed ones using time-demultiplexing technique,which are subsequently converted into electrical domain and filtered for anti-aliasing.Finally,the output time-interleaved signals are digitized by using the low-speed and high-resolution electronic ADCs and reconstructed in digital domain,respectively.Numerical simulation and experiment are carried out to demonstrate the feasibility of the proposed scheme.In the proof-of-concept experiment,the cavity-free optical pulse source is achieved with repetition rate and pulse width of 8 GHz and 5.7 ps,respectively.The output optical pulse train undersamples the input RF signal within 40 GHz and is time-demultiplexed into two channels with a lower sampling rate at 4 GS/s via a high-speed electro-optic switch.Finally,an effect number of bits(ENOB)more than 5.6 bits is acquired after quantization,code and timeinterleaved reconstruction.(3)In order to make the best use of optical sampling and overcome the speed limitation of electronic quantization,two ultra-fast all-optical quantization schemes based on soliton self-frequency shift(SSFS)and chirp compensation are proposed.A highresolution all-optical quantization scheme is proposed by employing a reflective loop.Through using n sections of SMF and n sections of high-nonlinear fiber(HNLF),a singlestage SSFS and(2n-1)-stage comb-like spectral compression based on positive and negative chirp compensation are achieved.The proposed quantization scheme achieves a simpler architecture,a larger optical compression ratio and a higher quantization resolution simultaneously compared with those of its uni-directional counterpart.The proposed scheme is evaluated by both numerical simulation and experiment in the case of n=2.In the experiment,a quantization resolution of 6.2 bits is obtained within the central wavelength shift from 1580.0 nm to 1672.2 nm,which is 1.2-bit higher than that of its uni-directional counterpart.In addition,a low-pedestal all-optical quantization based on a Sagnac loop is also proposed.The spectral compression architecture is mainly composed of one section of SMF and a Sagnac loop,where the Sagnac loop consists of an optical coupler and one section of HNLF.By setting the coupler ratio properly,the spectra are compressed through anomalous GVD effect and power-dependent filter effect.Meanwhile,the pedestal accompanied with the imperfect chirp compensation is reduced,which is beneficial to improve the quantization resolution and avoid coding misjudgment.(4)Considering that the fibers in the spectral compression scheme based on chirp compensation have a fixed length combination and possess a large size,a novel all-optical quantization scheme based on SSFS and time-dependent filtering is proposed.The multicolor optical pulses after SSFS are fed into a loop of dispersive fiber to realize wavelength-to-time mapping through GVD effect.Then,the pulse edges corresponding to the short-and long-wavelength components with low power are mainly filtered out,meanwhile the pulse center corresponding to the central wavelength components with high power passes by exploiting the nonlinear polarization rotation(NPR)effect in a spool of HNLF and polarizers to achieve power-dependent filtering.Thus,the spectral compression is obtained to improve the all-optical quantization resolution.Both numerical simulation and experiment are implemented to demonstrate the feasibility of the proposed scheme.The simulation results indicate that the proposed scheme can accommodate various optical pulses,are insensitive to the PC status and does not have a restrictive requirement of the symbol and the length of the dispersive fiber.In the experiment,the spectra after SSFS are efficiently compressed to an average width of 1.65 nm in a wavelength shift range of 100 nm,corresponding to a quantization resolution of5.95 bits,which is 1.13-bit higher than that of the conventional comb-like fibers scheme.