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Study On High Repetition Rate And Broadband Femtosecond Fiber Lasers

Posted on:2016-11-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:X LiFull Text:PDF
GTID:1108330503993842Subject:Electromagnetic field and microwave technology
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Femtosecond fiber lasers are desirable in many applications including photonic analog-to-digital conversion(PADC), femtosecond laser frequency comb generation, arbitrary optical waveform generation, optical coherence tomography, etc. In addition, more applications are emergying due to their unique advantages of compact design, low cost, easy operation, high energy conversion efficiency, and excellent stability. Up to date, the study of femtosecond fiber lasers are aiming to meet the demand of practical applications. High repetition rate and ultra-short pulse fiber laser is thereby a hot topic in this field.This dissertation mainly focuses on optimizing the output pulse spectrum of the femtosecond fiber laser and enhancing its overall performance in terms of repetition rate, spectrum width, pulse width and timing jitter, etc. The main content and the achievement obtained are as follows:Firstly, a model for numerical simulation based on the physical model of passively mode-locked fiber laser is built. The numerical simulations are carried out for mode-locked fiber lasers based on nonlinear polarization rotation(NPR) and hybridly mode-locked fiber laser cooperatively based on NPR and semiconductor saturable absorber mirror(SESAM). The evolution of the pulse time domain waveform and spectrum in the building process of mode-locking, the pulse waveform, and output spectrum shape after mode-locking, and the evolution of pulse and spectrum circulating in the cavity are simulated and compared, respectively.Secondly, this dissertation demonstrates a NPR-based mode-locked femtosecond fiber laser under the collaborative optimization of dispersion and nonlinearity. The fundamental repetition rate is larger than 200 MHz. The maximum power of the direct output pulse is 84.8 mW with a pulse duration of 56.5 fs, and the single-pulse energy of 0.42 nJ, and the pulse peak power of 7 k W. The transfer efficiency exhibited is as high as 12.1%. Evolution of the spectral characteristics of the direct output pulse is further studied by adjusting the waveplates and tuning the pump power. 44.6-fs ultrafast pulses with a flat spectral(full width at half maximum, FWHM) of 148 nm are directly generated after the fiber laser is optimized, which is very promising in applications such as WDM-based optical sampling where parallel channels up to hundred may be required. Without sacrificing spectral characteristics, the shortest pulse width is 34.3 fs by using SF10 prism laid outside the cavity to dechirp the pulse.Thirdly, this dissertation experimentally demonstrates an effectively improvement of timing jitter in a broadband femtosecond fiber laser by use of an intracavity bandpass filter(BPF). The laser output performances without and with the BPF are compared. When the 40 nm BPF is employed, the timing jitter is reduced significantly by 42% from 29.9 fs to 17.4 fs(1 kHz-10 MHz) due to the spectral filtering effect, which depresses the fluctuations of the optical frequency mean position induced by the quantum noise. Experimental results are in good agreement with theoretical analysis. The repetition rate is 201 MHz, the pulse duration is 56.5 fs, and the FWHM is 61.4 nm when the BPF is incorporated into the cavity. This high repetition rate, low timing jitter, broadband fiber laser provides an excellent source for high-speed and high-resolution PADC system.Fourthly, this dissertation demonstrates a hybrid scheme of Er-doped mode-locked fiber laser that incorporates both NPR and SESAM. The NPR is intended for shaping the ultrashort pulses while the SESAM serves as self-starting and stable mode locking. The repetition rate of the mode-locked output pulse is 212 MHz and the FWHM of the spectrum is 61.4 nm. The pulse duration is 41.9 fs, which is near the transform-limited range. No pedestal beats or sub-pulses are observed in both sides of the time domain pulse. The average output power is larger than 65 mW at a pump power of 750 mW, corresponding to a single-pulse energy of 0.31 nJ. Experimental results are in good agreement with theoretical analysis. Mode locking is self-starting and stable over 30 hours. This high repetition rate and ultra-short pulsed fiber laser could be an excellent source for the PADC and frequency comb generation.Finally, this dissertation demonstrates a tunable harmonic mode-locking(HML) fiber laser that incorporates both NPR and SESAM, which enables self-starting and stabilization of hybrid mode-locking. Meanwhile, the collaborative optimization of dispersion and nonlinearity are also carried out. The HML fiber laser could operate with repetition rates from the 2th harmonic(167 MHz) to the 8th harmonic(667 MHz) by increasing the pump power and adjusting the waveplates. For the highest repetition rate(the 8th HML), the FWHM is 181 nm and the output pulse duration is 218 fs, which is externally dechirped to 91 fs. The average output power is 136 mW and the SMSR is more than 60 dB. The 4th HML pulses at 333 MHz repetition rate are also measured with minimum compress pulse duration of 73 fs. Under the condition of stable HML, laser operation is maintained for nine hours with negligible pulse variation. This high repetition rate and broadband fiber laser could be an excellent source for PADC, biological imaging, and high-resolution optical coherence tomography.
Keywords/Search Tags:femtosecond fiber laser, erbium-doped fiber laser, ultrafast pulse, high repetition rate, broadband spectrum, timing jitter, bandpass filter(BPF), nonlinear polarization rotation(NPR), semiconductor saturable absorber mirror(SESAM), hybrid mode-locking
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