Spectral Regulation Of Femtosecond Optical Parametric Oscillators

Optical parametric oscillators（OPOs）are based on second-order nonlinear gain crystals.The bandwidth and central wavelength of the parametric gain of the nonlinear crystals can be tailored.The spectrum of femtosecond OPOs can be regulated by adjusting the intra-cavity dispersion,self-phase modulation effect,loss and temporal walk-off.Presently,femtosecond OPOs can be configured to generate ultra-short pulses with broad bandwidth or ultra-wide wavelength-tuning range,which can be used in biological imaging,molecular detection and ultra-short pulse generation.However,there are still many limitations in spectral regulation of femtosecond OPOs.The spectral regulation of femtosecond OPOs is also restricted by the phase-matching bandwidths of nonlinear crystals and the spectral coverage of dielectric mirrors.We conduct research to solve these problems.The primary contents of the dissertation are as follows.（1）The scheme of intra-cavity spectral broadening is introduced to chirped-pulse OPOs,resulting the elimination of the restriction of phase-matching bandwidth of nonlinear crystal on output spectrum bandwidth.Numerical simulations of spectral evolution are performed to analyze the effect of intra-cavity group delay dispersion on intra-cavity spectrum broadening.Experimentally,by inserting a 2-mm-long Zn Se plate into the OPO cavity to provide sufficient self-phase modulation,signal pulses with an instantaneous bandwidth of 20 THz（at-20 d B）,twice as much as the parametric gain-bandwidth of the nonlinear crystal,were obtained from an OPO based on a 3-mm-long periodically polarized Lithium Niobate crystal.（2）The concept of normalized nonlinear parameter of nonlinear crystals is proposed and the influence of the nonlinear parameters of nonlinear crystals on the intra-cavity spectral broadening in femtosecond OPOs is analyzed.Chirped-pulse OPOs based on a single nonlinear crystal with high nonlinear parameter can provide adequate self-phase modulation effect and realize intra-cavity spectral broadening,and therefore no additional nonlinear medium is required.The intra-cavity spectral regulation could be achieved by optimizing the intra-cavity second and third order dispersion,and output pulses with broad and flat spectrum could be obtained.Experimentally,an OPO system based on a KTA crystal is constructed,and a signal wave with a-20 dB bandwidth of 45.3 THz,around 12 times as much as the gain-bandwidth of a 3-mm-long KTi OAs O₄ crystal is obtained.The pulse width of generated signal pulses is then compressed to 52 fs outside the OPO cavity.（3）A unique femtosecond-OPO cavity architecture composed mainly of metal mirrors is proposed,which breaks the limitation on spectral coverage from dielectric mirrors.Stable resonant waves with a tuning range covering the signal to idler wavelengths are obtained.The reasons for multi-band and non-stationary oscillation of such a wide wavelength-tuning range OPO system is discussed.The spectral regulation is realized by intra-cavity dispersion management,and the stationary and single-band output within a very wide wavelength-tuning range is obtained.Experimentally,resonant waves with a tuning range over 652～2289 nm,covering nearly the full phase-match range of a lithium triborate crystal,are obtained.In conclusion,two kinds of femtosecond OPOs schemes are proposed for regulating the output spectrum,which are intra-cavity spectral broadening and ultra-wide resonant wavelength tuning respectively.The effect of different cavity parameters on the output spectra is analyzed.By using the intra-cavity spectral broadening scheme,the pulse evolution mechanisms and intracavity spectral broadening in femtosecond OPOs based on low normalized nonlinear parameter crystals and high normalized nonlinear parameter crystals are analyzed respectively.Output pulses with very broad bandwidths are obtained,exceeding the phase matching bandwidth of nonlinear crystals.The widely tunable OPO cavity architecture,which is composed mainly of metal mirrors,provides an alternative scheme to extend the spectral coverage of OPOs.The spectral regulation is realized by intra-cavity dispersion management,and the stable resonant waves with ultra-wide and continuous wavelength-tuning range are obtained.