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Research On Generation Of Broadband High-Repetition-Rate Pulses Based On Time Lens

Posted on:2019-01-02Degree:MasterType:Thesis
Country:ChinaCandidate:K R LiFull Text:PDF
GTID:2348330542498149Subject:Electronic Science and Technology
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For some applications related to optical signal processing,such as frequency metrology,ultrahigh-speed optical transmission,dense wavelength-division multiplexing and ultrafast optical sampling,broadband pulses at high repetition rates are desired.Mode-locked lasers are often used as pulse sources because of their stability and simple structures.Mode locking includes passive mode locking and active mode locking.Passive mode-locked laser can generate broadband pulse train with short durations.However,due to the factor of cavity length,the repetition rate is limited.Active mode-locked laser can easily emit high-repetition-rate pulses,but generally working at soliton pulse shaping regime,the pulse width is large and the spectral width is narrow.Relying on strong fiber nonlinearity,the active mode-locked laser can change to dissipative soliton and generate pulse train with broad spectrum.When increasing the repetition rate,fiber nonlinearity would be weakened as the peak power of pulse would be reduced.Coupled opto-electronic oscillator(COEO),which outputs high-rep-rate pulse train with very low noise,is developed based on the active mode-locked laser.The pulse shaping process in COEO is similar to active mode-locked laser.However,due to the generally short fiber in COEO,the accumulation of nonlinearity is insufficient as active mode-locked laser.Furthermore,even if the demand of nonlinearity can be met,for the ultra-short pulse,the perturbations from nonlinearity and dispersion would induce wave breaking.A COEO structure employing an intracavity time lens is investigated in this paper,which can generate high-rep-rate pulses with very low noise and meanwhile use the time lens to replace nonlinearity to broaden the spectrum.Based on structure of COEO,it gets rid of the dependence of the external driving source on active mode-locking,and generates high-rep-rate pulses whose noise is much lower than the active mode-locked laser.A phase modulator(PM)acting as a time lens is used to impose a quadratic phase onto the pulse in the laser cavity.Combining with cavity dispersion,the pulse is in a stretched state with a large bandwidth.The stable solution of this structure is analyzed with time domain ABCD matrix.The result shows that strong phase modulation and small dispersion would be good for generating pulse with wide spectrum and short duration.Then this scheme is numerically studied.A 10-GHz pulse train with 3-dB spectral width of 10.99 nm is achieved and then can be de-chirped and compressed down to 492.22 fs although without the participation of fiber nonlinearity.The output phase noise is also analyzed in this paper.The simulated single-side band(SSB)noise at 10-kHz offset frequency is about-140 dBc/Hz,demonstrating a nice noise characteristic.Compared to the COEO without time lens,the simulation shows that time lens contributes little to final SSB noise under strong phase modulation and small dispersion.The theoretical analysis and comparison simulation result illustrate that optimization of this scheme is also toward the direction of strong phase modulation and small dispersion.
Keywords/Search Tags:time lens, active mode-locking, coupled opto-electronic oscillator, fiber nonlinearity, high repetition rate, large bandwidth, low noise
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