| Femtosecond doubly-resonant optical parametric oscillators(DROs)could be applied in generating ultra-short pulses/broadband spectra,tunable lasers and quantum calculations potentially.The signal and idler lights are both oscillating in DROs.DROs have some unique features in terms of threshold,wavelength-tuning and phase-matching.There are many regimes of operation in DROs.The broadband phase-matching bandwidth supports many applications of DROs such as ultrafast pulses generation,broadband spectra generation and tunable lasers.This thesis is focusing on the regimes of operation in DROs based on periodically-poled lithium niobate(PPLN)crystal,and aims to solve the existing problems in the applications above and better utilize the broadband phase-matching bandwidth of DROs.The main works of this thesis are summarized as follows:(1)The influences of second/third-order dispersion,pump power and the length of the nonlinear crystal on the simultaneous bright-dark soliton regime in femtosecond DROs have been investigated in this thesis.Besides,the multi-pulses generation regime in femtosecond DROs was discovered.Specifically,based on numerical simulation,we illustrated that the third-order dispersion would limit the minimum pulse width of the generated signal light.We also discovered the multi-pulses generation regime and analyzed it theoretically.In the experiment,we have observed double-and triple-pulses in a PPLN-crystal-based DRO successfully.The research above has proved that residual third-order dispersion would limit the minimum pulse width generated in DROs working on the simultaneous bright-dark soliton regime and also clarified the pulse formation mechanism in DROs when the pulse width was limited with theoretical and experimental works.(2)A wavelength tuning regime inside the oscillation peaks of DROs has been proposed.Based on this regime,a continuously wavelength-tunable femtosecond DRO was demonstrated.Specifically,based on the phase-matching theory in DROs,we have given the relationship between the frequency shift of signal/idler waves and the cavity-length detuning.In the experiment,based on an optimized cavity-length locking system,we have realized the continuous wavelength tuning inside the oscillating peak and achieved a total wavelength tuning range of 1910～2340 nm with a 3-mm-long PPLN crystal.The research above has given a theoretical explanation of the continuous wavelength-tuning inside the resonance peak in femtosecond DROs and also successfully demonstrated a continuously tunable femtosecond DRO in the experiment.(3)The quasi-stationary state of near-degenerate femtosecond DROs under high pump levels has been researched.We have found that there were some unstable states with ultrabroadband output in the DROs under high pump levels.The power fluctuations of these broadband outputs mainly occurred at a high-frequency range and could be suppressed greatly with an averaging procedure.In the experiment,a 1.54～2.78 μm@-20 d B quasi-stationary spectrum that has fully covered the phase-matching bandwidth of a 3-mm-long PPLN crystal was generated.The research above has discovered the quasi-stationary state in femtosecond DROs under high pump levels and analyzed the spectra and power-noise features of DROs under this state.Based on a quasi-stationary femtosecond DRO,an ultra-broadband spectrum has been generated.In summary,this thesis has researched the novel regimes of operation in femtosecond DROs based on PPLN crystal,including simultaneous bright-dark soliton regime and multipulses generation in it,wavelength-tuning regime inside the oscillation peak and quasistationary operating regime.These regimes help utilize the potential of femtosecond DROs in ultra-short pulses generation,tunable lasers and ultra-broadband spectra generation.The regimes reported in this thesis could also be utilized in DROs based on other nonlinear crystals and be applied to spectroscopy,optical ranging,high harmonic generation and other fields. |
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