| Ultrashort pulse lasers,which are generally defined as laser pulses with durations in the scales of femtoseconds to a few tens of picoseconds,have the characteristics of extremely short pulse duration and huge peak power.Nowadays,ultrashort pulse lasers play important roles in the fields of various scientific researches and applications,such as bioimaging,material cutting,micro-nano fabrication and laser medicine.Mode-locked erbium-doped fiber lasers(MLEDFLs)are ideal light sources for promoting and popularizing the applications of ultrashort pulse lasers since they have the advantages of low-cost,stable performance and easy to use.While the requirements for the laser wavelength are different in different applications,the wavelengths of MLEDFLs are generally confined to a range of 1520 nm-1600nm.MLEDFLs will be more useful if their waveband can be efficiently converted to other wavebands needed.This is exactly the purpose of this thesis:extending the output waveband of MLEDFLs by using nolinear optical techniques.Firstly,the MLEDFLs were studied and built for nonlinear optical application.A supermode interference filter based on a seven-core fiber was proposed for tuning the wavelength of an erbium-doped fiber laser mode-locked by the nonlinear polarization rotation method.The wavelength of the generated femtosecond laser can be continuously tuned from 1538.8 to 1561.2nm.A narrow-band filter was used for tuning the wavelength of an all-polarization-maintaining erbium-doped fiber laser mode-locked by a carbon nanotube saturable absorber.The wavelength of the generated picosecond pulses can be tuned from 1522.6 to 1573.7 nm.Then,using MLEDFLs as the input seed laser,nonlinear optical techniques were utilized to extend the output laser waveband using different nonlinear media(nonlinear crystals and highly nonlinear fibers).1?Using a femtosecond Er-doped fiber laser as the input seed and a nonlinear crystal as a medium,780 nm band femtosecond pulses were generated based on the frequency-doubling The 1560 nm femtosecond pulses generated from an all-polarization-maintaining amplification system were used as the input seed and a temperature controlled MgO:PPLN crystal was used as the frequency doubler.The generated 780 nm waveband femtosecond laser has an average output power of 1.1 W with a pulse width of 183 fs and the power fluctuation is less than 0.5%during 24h.2?Using highly nonlinear fibers as media,three kinds of ultra-short pulse laser generation systems were made with all fiber structures.(1)Using a femtosecond Er-doped fiber laser as a seed,synchronized ultrashort pulses at two different wavebands were generated based on supercontinuum.Ultrashort pulses at 1.03?m and 1.93?m were generated separately.Both of them were synchronized with the 1.56?m laser.Furthermore,supercontinuum at the 2?m waveband was generated with high spectral flatness.(2)Using a picosecond Er-doped fiber laser as a seed,multi-wavelength ultrashort laser pulses were generated based on the fiber optical parametric oscillator and cascaded four wave mixing.17 lasing wavelengths were simultaneously achieved,among which 11 wavelengths from 1338 to 1705 nm have an SNR>30 dB.(3)Using a picosecond Er-doped fiber laser as a seed,dual-wavelength ultrashort laser pulses at 1.6-1.8?m band were generated based on stimulated Raman scattering and four wave mixing.In conclusion,nonlinear optical methods,including frequency doubling,supercontinuum,fiber optical parametric oscillator,four wave mixing,and stimulated Raman scattering,have been exploited to extend the laser waveband of MLEDFLs. |
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