Research On Key Technologies Of Optical Performance Monitoring Based On All Optical Sampling

Optical fiber communications are developing towards the directions of ultra-highcapability, ultra-high speed and ultra-long distance, and the system is more and morecomplicated. As a result, it is important to monitor the quality of optical signals to ensuresafe and reliable operation. Traditional electronic sampling based monitor methods arelimited by electronic bottleneck, and thus cannot analyze high speed signals. However, alloptical sampling techniques break through electronic bottleneck and have the advantages ofrate and modulation format transparent. Therefore, the prospect of application is bright.In this paper, several key technologies on all optical sampling based opticalperformance monitoring were investigated, including:(1)The algorithm of asynchronous software synchronous optical sampling wasdiscussed. There is no need to obey Nyquist sampling theorem, so low speed electricaldevices could be used. When frequency reduction remainder is zeros or its reciprocal isfinite decimal, the quality of recovered eye diagram is poor, which is called the inherentdrawback of software synchronous optical sampling. Besides, the requirement of amplitudejitter, timing jitter and width of sampling pulses was investigated, which is helpful for thedesign of sampling pulse source.(2)The model of nonlinear polarization rotation mode-locked fiber laser was built,where Jones matrix was used to describe polarization controllers and Giles model was usedto describe Erbium doped fiber. Compared with existing models, the model introduced hereis more close to real components, and physical meaning is clear. According to simulationresults, full width half maximum of the pulse is622fs, the width of the spectrum is4.8nmand there is strong sideband, which means the pulses are typical solitons. With the help ofthis model, how the angle of the polarization controllers and pump power influence thepulses was investigated. The result showed that random birefringence decreases the tuningrange of polarization controllers, the average power of pulses is in proportion to pumppower and pulses would not form if the pump power is too high or too low. The mode-lockfiber laser was investigated experimentally and29.54MHz sampling pulses were generated,with the central wavelength of1575.91nm and the width of630fs.(3)The second harmonic generation(SHG) effect and sum frequency generation(SFG)effect in periodically poled lithium niobate waveguide were investigated, and SFG based optical sampling system was designed. Band pass filter(BPF) was used before the samplingpulses were sent into the Erbium doped fiber amplifier. Compared with the system withoutBPF, the peak power of amplified sampling pulses is10dB higher. The influence of thewidth of BPF on SFG light and SHG light was discussed, which showed that when thewidth is0.4nm the quality of eye diagram is best. Optical sampling oscilloscope prototypewas built and according to the test result, the eye diagram of12Gbit/s intensity modulatedsignal could be recovered, the sensitivity of the prototype is-3dBm, and the measuredsignal characters such as rate, optical signal to noise ratio and dispersions are coincidentwith commercial high speed electrical sampling oscilloscope.(4)In order to realize automatic measurement, a method was introduced as follows.When the wavelength of sampling pulses is tuning, the peak of the standard deviation ofsampled signal is found, and the corresponding wavelength is the quasi-phase-matchingwavelength. Both simulated and experimental results showed that the standard deviation ofsampled signal is in proportion to the power of SFG light while is not affected by SHG light,and thus the feasibility of this method was proven. Polarization division structure wasdesigned, and the polarization dependence of SFG light was1.67dB. Eye diagrams ofoptical signals with the wavelength from1530to1656nm were derived by use of widespectrum pulses.(5)Traditional software synchronous optical sampling technique could not recover therate of the signal. In this paper, sampling pulses with different repetition frequencies wereused to sample the signal, so the rate could be derived according to correspondingalgorithm. A cavity length tunable sampling source was made, and mode-locked pulses withthe repetition frequencies of20.26677MHz and20.22900MHz were generated. Therecovered rate of100MHz sinusoidal signal was102.22MHz. Besides, two blind zones ofthis method were firstly investigated. By use of a third sampling pulse, the blind zone waseliminated, which was testified by theoretical analysis, simulation and experiment.(6)Linear optical sampling technique was investigated. The expression of linearsampled signal was given, and the methods to eliminate frequency deviation, phasedeviation and phase noise were discussed. With the help of optical hybrid and balancedetectors, linear sampling system was constructed. The waveform of continuous wave andthe eye diagram of12Gbit/s intensity modulated optical signal were recovered, and theconstellation of1MHz2PSK signal was also observed.